What Are the Challenges Posed for Underbalanced Drilling Operation - Coursework Example

What are the challenges posed for underbalanced drilling operations? Executive Summary There has been a significant change in the manner in which drilling operations are now being carried out in oil and gas explorations. The normal and usual mechanism of overbalance drilling is being slowly replaced by underbalanced drilling because of the numerous advantages this process offers. If these operations are carried out scientifically and as per designed guidelines, most of the shortcomings of overbalanced drilling operations such as mud formation, mixing of circulating fluids and other impurities getting trapped in the drilled oil stand to be eliminated. More research is taking place in this field to make the underbalanced drilling operation more robust since a substandard underbalanced drilling operation has the potential to cause more damage than a poorly executed overbalanced drilling operation. This paper addresses the challenges posed in underbalanced drilling and suggests screening criteria for selecting areas where these operations can be effectively performed. Introduction The main intent of looking for others forms of drilling is to address the issues and concerns that overbalanced drilling brings about. Since most formations are in an overbalanced state, these are usually drilled using water based high density fluids. Such a situation creates an overbalance pressure, which is a sum of the hydrostatic pressure of the liquid column and pressure transmitted by the pump to the circulating fluid. (Thomas F.B and Bennion D.B, 1994) This pressure exceeds the inherent pressure in the rock formation thereby resulting in the upward movement of oil through the drilled crevice. Some of the anomalies which underbalanced drilling seeks to avoid are mixing of mud with oil thereby forming sludge emulsions. The incompatibility between drilling fluid and soil formations like smectitic clay or kaolinite clay has the tendency of swelling the rock formation thereby reducing its permeability characteristics. (Thomas F.B and Bennion D.B, 1994) Artificial agents in the drilling fluid or drill bits that might get entrapped in the fluid have the tendency of reducing permeability. Another concern is the entrapment of mixed oil water phase near the wellbore that reduce oil production significantly. Sometimes chemical additives are mixed in the drilling fluid to generate more impact. This however has the tendency of getting adsorbed into the system thereby reducing oil flow. (Thomas F.B and Bennion D.B, 1994) Source: Thomas F.B and Bennion D.B, Schematic representations of solid and fluid losses in underbalanced and overbalanced operations, 1994 Highly overbalanced conditions have a tendency to cause loss of large amount of liquids like water that might be trapped in the rock formations. This sudden loss also causes particulates within this region to migrate thereby causing a kind of vacuum for the formation to cave in. Aerobic and anaerobic bacteria have the tendency of reacting with these liquids and forming polysaccharide which reduce porosity in the region. (Thomas F.B and Bennion D.B, 1994) These defects can be overcome to an extent if a quality underbalanced drilling is performed. What is underbalanced drilling? Underbalanced drilling is carried out when the pressure of the downward circulating drilling fluid is less than the inherent pressure in the rock formation. While in some areas these conditions occur naturally, in others, such a state can be created by the use of hydrocarbon fluids of lesser densities. (Thomas F.B and Bennion D.B, 1994) This underbalanced condition can also be maintained by circulating along with the nitrogen or natural gases to keep the density at low value. Sometimes tubing strings are utilized to inject the gas and keep the circulating fluid density in check. Using such non-condensable gases to lower the density of circulating fluid is also called as ‘artificial’ underbalanced drilling. The Process Before proceeding to the advantages and disadvantages of underbalanced drilling it is very important to decipher what exactly this process entails and how it varies with overbalanced drilling operations in the actual execution of the process. Source: Engevik Oma Mari, Illustration of bottom hole pressure during OBD & UBD operations, 2007 Use of different densities of liquid creates different pressure at the bottom hole. It is noted from the figure that with the use of low density fluids with top side pressure, the required bottom hole pressure is obtained at the same time keeping a slender drilling window which basically means that higher pressures are obtained keeping the bore hole window at a minimum. (Engevik Oma Mari, 2007) This is clearly an advantage since narrow bottom hole windows means the chances of the rock formation capsizing are minimum. Source: Engevik Oma Mari, Pressure margins in UBD & OBD system, 2007 During overbalanced drilling operations the bottom hole pressure should be lower than the fracture pressure of the bottom rock formation since pressure in excess can lead to it capsizing and drill fluid being lost into the fractures. (Engevik Oma Mari, 2007) However this bottom pressure achieved is usually higher than the pore pressure. As this pressure goes below the pore pressure, the oil from the rock formation slowly makes its way into the wellbore. Comparing this to an underbalanced drilling operation, the pressure achieved or maintained is normally below both the fracture pressure and the pore pressure as shown in the diagram. (Engevik Oma Mari,2007) Hence minimum amount of control is required for the influx of oil into the wellbore since the bottom hole pressure is always less than the pore hole pressure and this condition is naturally maintained. The only mishap that might occur is the bottom hole pressure going way below and approaching collapsing pressure. This will lead to an excess influx of oil which sometimes has the capability of destroying the well formation. However the chances of the bottom hole pressure exceeding the fracture pressure and the well bore capsizing is minimal. Advantages of underbalanced drilling Underbalanced drilling can be effectively carried out in areas where there is high incidence of fractured sandstone, vugular carbonates and high permeability sands. (Bennion Brant, 1997) Some of the common advantages of underbalanced drilling include 1. Formation damage is reduced considerably. 2. There is no possibility of circulation in flow getting hampered. 3. Systems for disposing of the mud generated can be dispensed with thus saving a lot of money. (Bennion Brant, 1997) 4. The drilling costs also tend to go down significantly due to the increase in life of the drill bit. 5. The process is reliable and can be better controlled. Areas where production can be increased can be ascertained during the drilling process. This leads to better planning. 6. Flow test can be performed while drilling thus keeping a better control over operations and the quality of oil drilled. 7. ROP defined as the rate of penetration is sometimes better achieved in certain rock formations with underbalanced drilling. (Engevik Oma Mari, 2007) This reduces the drilling time significantly and thus lot of money is saved in adopting this process. 8. For depleted and fractured reservoirs underbalanced drilling offers the best option of drilling out remaining oil. Disadvantages of underbalanced drilling Some the disadvantages that have been highlighted in underbalanced drilling operations include 1. Oil reservoirs containing hydrogen sulphide run the risk of being susceptible to fire and explosions. 2. Nitrogen is considered as an optimum gas for carrying out underbalanced activities because of its inert nature. However continuous use of nitrogen is a costly process. While air is a cheaper option, problems arise if combustion occurs between the oxygen in air and the oil stream. (Bennion Brant, 1997) 3. Loss of underbalanced conditions can lead to pulses of high pressure causing hydrostatic Colum effects, depletion problems, damage to bits and problems to other piping connections. 4. Apart from these, there is a tendency of potential damage in reservoirs if the underbalanced drilling process is not efficient. (Engevik Oma Mari, 2007)) These damages are discussed in detail in the next section. a. Absence of a protective sealing filter cake means that if underbalanced conditions are not maintained throughout, it might lead to mud logging. Entry to extremely low pressure areas and technical failure in the supply of gas might lead to shutdown of the process leading to loss in underbalanced conditions. (Thomas F.B and Bennion D.B, 1994) Recent use of coiled tubing and concentric strings has however improved the situation and lead to optimum conditions being attained during drilling operations. b. Imbibitions can cause water to surge up several hundreds of metres while drilling. This is an effect of capillary action which causes water to rise up due to the presence of differential pressure. (McCaffery F.G, 1973) The well bore region thus gets completely submerged in water and cause total blockage. c. Glazing and surface damage are also concerns in underbalanced drilling. This happens when the specific heat capacity or in other words the capacity of the fluid to absorb heat is less which results in local temperatures getting exceedingly high. (Thomas F.B and Bennion D.B, 1994) However use of parasitic strings has managed to control this aspect to a large extent. Screening Process for underbalanced drilling There is screening process that needs to be initiated and data needs to be gathered according to a set of parameters to establish whether underbalanced drilling operations can be conducted in a formation. Some of the formations where this is possible include reservoirs which have lost circulation and that have permeability in the range of 1000md, carbonate structures with vugular porosity or zones where overbalance pressures of 1000 psi exist. (Bennion Brant, 1997) Underbalanced drilling can be used in these areas since overbalanced drilling is quite difficult to sustain in these conditions. Such kind of conditions can often cause the drilling pipe to get stuck due to the detrimental effect of muddy sludge formations. The screening process for selecting a formation for underbalanced drilling includes the following 12 steps. (Bennion Brant, 1997) 1. The initial step is gathering all information regarding the formation so that this data can be studied and analysed for possibility of underbalanced drilling. 2. This data is then presented in an intelligent format to reservoir engineers, drilling technicians and experts on geology to make an initial assessment and get their feedback as to whether the formation meets the criteria for underbalanced drilling. 3. A detailed study is then carried out with the information collected and the team is widened to include not just core technical personnel but also specialists in geophysics, petro physics, consultants in productions engineering, laboratory people, safety team and service companies that would be involved in drilling. (Bennion Brant, 1997) 4. There might be several techniques to maintain an underbalanced condition or create an underbalanced environment. All pros and cons should be discussed and most optimum chosen. 5. The next step is critical as it involves choosing the right team to execute the task. 6. Once the team is fixed a meeting is held to delegate responsibilities and tasks to all assigned personnel so that there is no confusion on that front. 7. Any new equipments if need to be procured are done and transported to site. These are then set up at site and tests are carried out to ensure it is in fine working condition. 8. The underbalanced drilling operations are slowly put into action and data that is acquired during real time is also assimilated and fed back to the experts. This would help them assess the information and analyse whether any modifications in the process is required. (Bennion Brant, 1997) 9. This data is also further used in getting a real time idea of the soil structure when the drilling operations proceed further into the rock formation. Any safeguards that would require to be taken in case of any unexpected occurrences are also taken into account. 10. All steps during the actual process are taken to ensure that the drilling into the rock formation takes place in the underbalanced conditions without losing process parameters. 11. A review of the process after the completion is carried out and a self assessment is done as to how effectively the drilling process was carried out. 12. The next step involves the beginning of the production phase. Reviews are also carried out post the production phase. This effectively completes the parameters followed in underbalanced drilling. (Bennion Brant, 1997) Type of data required and used for analysing underbalanced drilling operations There is a whole range of data that are used to study whether a particular rock formation can be engaged in underbalanced drilling operations. (Bennion Brant , 1997) These are classified as a. Reservoir properties The real time down hole pressure The extent of pressure variation in a common rock formation reservoir. Zones within the same region that have different outputs and the pressure in the same. Contacts points of oil, gas and water Location of faultiness. b. Rock properties (Bennion Brant, 1997) Classification of soil strata Permeability of these layers The extent of porosity Size of these pores and its distribution within the rock formation. Presence of fractures Grain structure and presence of clay structures Strength of these formations Determinants like capillary pressure Degree of wetness and relative permeability of water, oil and gas Glazing characteristics c. Reservoir Fluid properties (Bennion Brant, 1997) The chemical composition of oil and gas Degree of paraffin and asphaltene in oil Pour point, aniline point and cloud point of oil Density and viscosity of the fluid at the well conditions Bubble point pressure and dew point pressure d. In-situ fluid and Drilling Fluid Compatibility (Bennion Brant, 1997) Potential to emulsify Potential of hydration Scale potential Tendency of precipitation formation Tendency to dissolve gases Potential of corrosion and explosion e. Rock and Drilling Fluid Compatibility (Bennion Brant, 1997) Negative reaction with clayey materials Tendency of rock formations to dissolve on contact with drilling fluid Potential for imbibitions. f. Base Fluid (Bennion Brant, 1997) Fresh water Inhibited brine The crude produced Refined crude oil Foam g. Density of reducing chemicals (Bennion Brant, 1997) Nitrogen Natural gas Air Air with reduced oxygen content Flue gas Low density glass spheroids and other liquids Equipment used in underbalanced drilling operations The underbalanced equipment used in drilling ensures that there is a positive UB ratio, the well bore is adequately sealed at the top and the equipment at the surface is capable of extracting the formation fluid to ensure a smooth operation without bottlenecking. The different parts of UBD equipment include rig circulating equipment, drill string, non return valves, Blowout preventer, several control systems, separators, flare stacks, flare pits and flare lines. (Engevik Oma Mari, 2007) The idea of maintaining such a comprehensive closed system is to limit pressures within permissible levels and also extract formation fluids in quantities that are manageable for the system. The UBD system is typically divided into two parts, the surface separation package system and the well system. The separation system includes pumps, separators and mud pits. The figure shows two separators with high pressure gas and low pressure gas separated in the first and second phase respectively. The test separator separates the oil part while the mud processing area separates the mud and transfers it to the mud pit. (Engevik Oma Mari, 2007) Source: Engevik Oma Mari, Illustration of a UBD system, 2007 The well system on the other hand consists of a drill string or parasite casing which is being drilled into the formation and allows the drilled fluid to rise up and flow into the surface separator system encountering the blow out preventer, the ESD valve and the flow spool on the way as depicted in the figure. Other parts of the system include rotating diverter which is the prime mover of the drill string. The rotating control head maintains perfect sealing around the drill string while rotating blowout preventer allows for smooth upward and downward movement of pipe. Apart from these parts the system also has surface and subsurface equipments. The surface equipments include rotating control device (RCD) which maintains and controls annulus pressure while drilling, 4 phase downstream fluid separation package, emergency shutdown system, pressure relief valves, geological sampling devices, mud pumps and metering devices. (Lunan B , 1995) Apart from these important devices it also has a chemical injection unit, and a choke manifold system. The subsurface equipment on the other hand consists of the drill string, bottom hole assembly and check valves at the subsurface level which prevent backflow of fluid into the drill string and also prevents return of gas when piping connections are established. (Engevik Oma Mari, 2007) Different methods of underbalanced drilling There are four methods of underbalanced drilling based on the kind of drilling fluid used 1. Liquid mud can be effectively used. The mix can be either of water or oil but devoid of any gases. The mix is homogenous, incompressible and maintains uniform density throughout the drilling operation. (Engevik Oma Mari, 2007) The mud is pumped using a drill string but these fluids sometimes become compressible when it gets mixed with the hydrocarbon that comes up. 2. The mix can be a gaseated fluid of liquid and gas. In rock formations having low hydrostatic pressure the mud is mixed with nitrogen, air and exhaust gas thus making is lighter. The liquid used in getting a homogenous mix is either oil or water based and a parasite pipe string or casing is used to introduce this into the rock formations. Another mix that falls in the same category is using gas for drilling with liquid being mixed into the gaseous stream. The concentration of such liquids is less than 2.5%. (Engevik Oma Mari, 2007) The liquid mist introduced in the process has an additional advantage that it is used in cleaning the surface of the drill bit along with removing small amount of debris surrounding the drill bit through the annular space. 3. Mixing a strong emulsifying agent with liquid gas creates stable foam which is a strong drilling agent. The percentage of nitrogen gas which is usually used varies between 55% and 97.5%. (Engevik Oma Mari, 2007) 4. The Arkoma Basin of western Arkansas and eastern Oklahoma has been successfully drilling using air or natural gas for the past 30 years. The gas passed through the drill string is air, nitrogen or exhaust gas. Once the surface of rock formation is entered the gas gets mixed with the oil and comes up back to the surface through the annulus in the form of suspended mist. This is the most common underbalanced drilling process followed around the world. There are many ways to circulate the drilling fluid in underbalanced drilling operations. a. The fluids are pumped through the drill strings. b. Using parasitic pipe string whereby a separate injection string is used to allow drilling fluid to enter into the parasitic string and which ultimately comes out through the annulus. (Engevik Oma Mari, 2007) c. Using a parasitic casing which allows space for a separate injection annulus. Risk Assessment of underbalanced drilling operations For all underbalanced drilling operations SAFOP (safe operability analysis) and HAZOP (Hazard Operability analysis) are carried out to assess the safety aspects of the process and look for any loopholes that need to be plugged and which might affect efficient operation of the system. (Engevik Oma Mari, 2007) Inputs from different specialists are ascertained and a systematic study of technical and errors due to human negligence are identified and measures to counter the same are proposed. It also introduces guide words that help in quick error identification and looks for potential hazards at critical areas of the system. The cons of the system though marginal include a dependency on the technical expertise of the person involved in conducting the study. The quality of the HAZOP study depends on the extent of data sourced and is often a time consuming process. The information produced as an output of the study is also quite extensive and often overlooks common failures in its quest to identify potentially greater hazards. A sample of a typical study and the responses it generates is discussed here. (Engevik Oma Mari, 2007) The step discussed here is the lifting of pipes 1. a. Guideword: Unclear b. Deviation: The pipes are in an erroneous position and the pipes along with equipment are damaged. c. Possible causes: The equipment used is not proper/ the force applied is incorrect/ position indicated is confusing d. Consequences: This leads to delay in operations with damaged pipes/ equipment needing to be replaced. e. Action taken: Identify speed, equipments and the correct position to be followed in lifting the pipe and incorporate the same in the procedure in a simple and lucid manner (Engevik Oma Mari, 2007) 2. a. Guideword: Wrong action b. Deviation: During the drilling process pipe gets lost in the bottom hole. c. Possible causes: Along with incorrect equipment used the force applied is either too high or too low. d. Consequences: Along with surges that might occur, the pipes/ equipment along with the drill bit too might get damaged. e. Action taken: Rectify the procedure 3. a. Guideword: Step Omitted b. Deviation: The pipes are in left in an erroneous position. c. Possible causes: The procedure has left out the details about the lifting procedure. d. Consequences: Pipes along with pipe rams and drill bits are damaged. e. Action taken: Immediately proceed to training personnel and updating the procedure to identify possible omissions. (Engevik Oma Mari, 2007) Underbalanced drilling: A Case Study Three oil wells were subjected by Saudi Aramco in Saudi Arabia to underbalanced drilling and results compared to gauge their performance. These wells were located in the Arab D carbonate region of Uthmaniyah Field and reservoir pressure in all three wells exceeded 2890 psi. (Muqeem Muhammed and Jarret Clark, 2008) Description of the wells The first well UBD Oil-1 drilled had parameters of 6 1/8 inch hole drilled for a lateral length of 1102 ft in 16 hours with the rate of penetration (ROP) at 69 ft/hr. Production was rated to be approximately 4679 BOPD. The configuration as depicted in the figure entailed a 9 5/8 inch vertical casing string along with a horizontal 7 inch casing string along with a Halliburton Quick Trip Valve (QTV). Problems of casing getting stuck were noticed during this operation. (Muqeem Muhammed and Jarret Clark, 2008) The second well UBD Oil-2 drilled had parameters of 5 7/8 inch pipe drilled for a lateral length of 1443 ft in 24.7 hours with the rate of penetration (ROP) at 58.3 ft/hr. Production was rated to be approximately 6912 BOPD. The configuration as depicted in the figure entailed a 9 5/8 inch vertical casing string along with a horizontal 7 inch casing string without a Quick Trip Valve (QTV). (Muqeem Muhammed and Jarret Clark, 2008) The third well UBD Oil-3 drilled had parameters of 6 1/8 inch pipe drilled for a lateral length of 1725 ft in 20 hours with the rate of penetration (ROP) at 86.3 ft/hr. Production was rated to be approximately 5856 BOPD. (Muqeem Muhammed and Jarret Clark, 2008) Source: Muqeem Muhammed and Jarret Clar, Well configuration of UBD Oil-1, 2008 Source: Muqeem Muhammed and Jarret Clar, Well configuration of UBD Oil-2, 2008 Source: Muqeem Muhammed and Jarret Clar, Well configuration of UBD Oil-3, 2008 H2S control H2S control gained prime importance as this was a serious safety concern for people working in the facility. It was noticed that H2S was not properly controlled in UBD Oil-1. With the experience gained in drilling for UBD Oil-1, Rawabi United Safety was instrumental in installing four passive scrubbers thereby venting large amounts of H2S from around the tank vicinity. However with the use of two more active scrubbers in UBD Oil-3, there was a significant reduction of H2S from the vicinity of the well. (Muqeem Muhammed and Jarret Clark, 2008) Challenges Faced For the UBD Oil-1, it was found out that the outer diameter of the tool might have been slightly on the higher side and was the prime reason for it getting stuck. The well was sidetracked and the corresponding area drilled again. Source: Muqeem Muhammed and Jarret Clar, UBD Vs Conventional Time & Cost Comparison, 2008 For UBD Oil-2, the hole drilled was expanded to 9 7/8 inches. The 7 inch casing therefore faced no problems of getting stuck and movement was smooth. However isolation valves maintained at the bottom hole were not effective in maintaining underbalanced conditions. (Muqeem Muhammed and Jarret Clark, 2008) Source: Muqeem Muhammed and Jarret Clar, Mechanical downhole isolation tool, 2008 For UBD Oil-3, most of the challenges were overcome including maintaining underbalanced conditions. However in some cases these conditions ceased to exist on account of debris getting entrapped in the flapper valve. Final analysis and lessons learnt a. Correctly assessing the amount of H2S that could be released is important to set up alarm trips. In the case of UBD Oil-1, the program was set for 700 ppm while the outlet witnessed a surge of 4000 ppm. (Muqeem Muhammed and Jarret Clark, 2008) b. The vent points located close to the system meant that more the amount of fluid that was being drilled, significant was the volume of released vapours. Scrubbers kept at outlets were capable of mitigating to a large extent the concerns of H2S health hazards. c. The study completed in May 2006 was able to shed valuable information regarding the degree of invasion damage in the three oil wells while at the same time maximising oil production. d. These oil wells dug using underbalanced technology in the Ghawar Field have been successfully producing oil with UBD Oil-1 generating 4679 BPD at a gas rate of 2.5 MMscfd. UBD Oil-2 had a output of 6912 BPD at 2.0 MMscfd while UBD Oil-3 had an output of 5856 BOPD at a gas rate of 2.6 MMscfd. None of these wells produced water as a by product of the drilling process. (Muqeem Muhammed and Jarret Clark, 2008) Source: Muqeem Muhammed and Jarret Clar, Hydraulically opened downhole deployment tool used in UBD Oil well-3 which eliminated need for hydraulic workover unit, 2008 e. Isolation valves were used in these wells to maintain underbalanced conditions. UBD Oil-1 and UBD Oil-2 used Halliburton Quick Trip valve but these posed several problems which were overcome when Weatherford Downhole Deployment valve was used in UBD Oil-3. Conclusion Underbalanced drilling operations have certainly received a major fillip due to a significant amount of research that is taking place in these operations with the feedback received from a variety of experts. The idea is to create a system where oil can be drilled from underground rock formations but with minimal invasion so that the quality of product sourced is higher. In this aspect, variety of drilling fluids with different mix creating different densities is being used to generate this drilling process. The challenge is to maintain a constant underbalanced environment since loss of the same can lead to severe reduction in permeability characteristics at the rock formation leading to depletion in oil from the well. The decision to go for a underbalanced drilling operation should be carefully studied, its pros and cons assessed and then a decision finalized. If the rock formation is homogenous and risk of potential fluid invasion is minimal it might be more suitable to go for an overbalanced drilling operation than risking for a costlier and technically more difficult underbalanced drilling activity. However considering all the pros and cons it would still be considered that a well engineered underbalanced drilling operation can cause minimum damage of rock formation at the same time maximizing production output. Bibliography Bennion Brant, Reservoir-Screening criteria for Underbalanced drilling, Harts Petroleum Engineer International, 1997, Engevik Oma Mari, Risk assessment of underbalanced and managed pressure drilling operations, 2007, Web. 20th January 2013. < www.ntnu.no/ross/reports/stud/mari_engevik.pdf Lunan B, Surface control systems for underbalanced drilling, Journal of Canadian Petroleum Technology, September 1995, Print McCaffery F.G, The effect of wettability, relative permeability and imbibitions in porous media, Phd Thesis, University of Calgary, September 1973 Muqeem Muhammed and Jarret Clark, Underbalanced drilling of oil wells in Saudi Arabia, 2008, Thomas F.B and Bennion D.B, Underbalanced drilling of horizontal wells: Does it really eliminate formation damage? Society of Petroleum Engineers, 1994, Web. 20th January 2013. Read More