Horizontal Underbalanced Drilling in Northeast Brazil: A Field Case History
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The biggest challenge of the aerated fluid drilling technology development was overcome with the well in Albacora being drilled with the semi-submersible platform Petrobras 17 (P-17). A previous paper1 described the stages before the drilling activity: planning, new equipment, rig modifications and all the preparation for drilling the well. This paper describes the drilling operation itself. All the important points and differences relative to the conventional drilling method are highlighted, with special emphasis to the new equipment installed at the platform: the vertical compact separator, the automatic control system, and the RiserCap™ rotating control head. The operation of the new equipment, performance and the problems observed are presented and discussed. The points of improvement are suggested and ways to move forward with this new technology also discussed in the paper. The present work also addresses the positive impact of this field test relative to the implementation of the light-weight fluids technology from floating units. This initiative has been carried out systematically by Petrobras in the form of a Joint Industry Project (JIP) and has been congregating operators, service companies and consultants. The next steps of this project to use effectively light-weight fluids for drilling in deepwater are discussed. The experienced obtained with this operation was crucial to direct and guide the most important points to be studied and developed in the next phase of the project. Introduction The injection of a liquid and gas mixture for drilling purposes is not new in the oil industry. Besides the recent trend towards the use of underbalanced drilling (UBD), the literature2–5 documents a considerable number of operations carried out for limiting loss of circulation, avoiding differential sticking or improving penetration rates in competent crystalline rocks. These traditional low-head drilling (LHD) methods, which conduct downhole pressures just above the reservoir pressure, focus on targets that are somewhat distinct from those aimed by the UBD approach. The drastic reduction or elimination of formation damage are indeed the major motivation for the use of the underbalanced concept, as discussed elsewhere6,7. However, despite the differences between LHD and UBD, they are both techniques in which the injection of a gas-liquid mixture plays an important role in the drilling process. The injection of gas and the treatment of the returning mixture certainly take part in the list of the most challenging activities to manage in the operations. These aspects are so demanding that they are responsible for having almost restricted the application of UBD and LHD to the onshore scenario. However, the widely announced success of many projects is motivating the industry to employ considerable efforts for making it available in the offshore environment where lack of space for accommodating new equipment is the main concern. Besides the demand for space, floating vessels request much more in terms of innovative design because they have a configuration of surface and subsea equipment that is completely different from land rigs, fixed platforms or jack-up units. In addition, it is always good to remember that the adequate solutions for floaters should be technically and economically viable without causing any impact to the operational risks. Petrobras, which has the majority of its reserves in water depths greater than 400 m, decided to develop competence to bring this technology to floaters. The venture successfully achieved all the goals8–13 and involved other operators and service companies in the form of a JIP. The present work reports the ultimate task of the whole project that was the LHD test performed in the well Albacora 64H in December 2000.
The biggest challenge of the aerated fluid drilling technology development was overcome with the well in Albacora being drilled with the semi-submersible platform Petrobras 17 (P-17). A previous paper1 described the stages before the drilling activity: planning, new equipment, rig modifications and all the preparation for drilling the well. This paper describes the drilling operation itself. All the important points and differences relative to the conventional drilling method are highlighted, with special emphasis to the new equipment installed at the platform: the vertical compact separator, the automatic control system, and the RiserCap™ rotating control head. The operation of the new equipment, performance and the problems observed are presented and discussed. The points of improvement are suggested and ways to move forward with this new technology also discussed in the paper. The present work also addresses the positive impact of this field test relative to the implementation of the light-weight fluids technology from floating units. This initiative has been carried out systematically by Petrobras in the form of a Joint Industry Project (JIP) and has been congregating operators, service companies and consultants. The next steps of this project to use effectively light-weight fluids for drilling in deepwater are discussed. The experienced obtained with this operation was crucial to direct and guide the most important points to be studied and developed in the next phase of the project. Introduction The injection of a liquid and gas mixture for drilling purposes is not new in the oil industry. Besides the recent trend towards the use of underbalanced drilling (UBD), the literature2–5 documents a considerable number of operations carried out for limiting loss of circulation, avoiding differential sticking or improving penetration rates in competent crystalline rocks. These traditional low-head drilling (LHD) methods, which conduct downhole pressures just above the reservoir pressure, focus on targets that are somewhat distinct from those aimed by the UBD approach. The drastic reduction or elimination of formation damage are indeed the major motivation for the use of the underbalanced concept, as discussed elsewhere6,7. However, despite the differences between LHD and UBD, they are both techniques in which the injection of a gas-liquid mixture plays an important role in the drilling process. The injection of gas and the treatment of the returning mixture certainly take part in the list of the most challenging activities to manage in the operations. These aspects are so demanding that they are responsible for having almost restricted the application of UBD and LHD to the onshore scenario. However, the widely announced success of many projects is motivating the industry to employ considerable efforts for making it available in the offshore environment where lack of space for accommodating new equipment is the main concern. Besides the demand for space, floating vessels request much more in terms of innovative design because they have a configuration of surface and subsea equipment that is completely different from land rigs, fixed platforms or jack-up units. In addition, it is always good to remember that the adequate solutions for floaters should be technically and economically viable without causing any impact to the operational risks. Petrobras, which has the majority of its reserves in water depths greater than 400 m, decided to develop competence to bring this technology to floaters. The venture successfully achieved all the goals8–13 and involved other operators and service companies in the form of a JIP. The present work reports the ultimate task of the whole project that was the LHD test performed in the well Albacora 64H in December 2000.
Accurate prediction of shut-in and flowing bottom hole pressures in inclined holes presents a challenge in aerated mud drilling. It is highly desirable to develop a simple and accurate hydraulics equation for this purpose. This paper fills the gap. A closed form hydraulics equation was developed in this study on the basis of recent experiments on multiphase flow in an inclined well model. The newly developed hydraulics equation is a 4-phase model considering injected liquid, injected gas, formation fluid influx, and cuttings entrained at bottom hole during drilling. The equation was first calibrated using data measured from a full-scale research borehole. It was then tested with two field cases covering a deep (9831 ft) vertical well and a shallow (697 ft vertical depth) horizontal well drilled with aerated muds. The results show 8.66% and 0.81% error of the equation in bottomhole pressure prediction for the deep and shallow well, respectively. The equation was also compared with two commercial software packages (S1 and S2) using measured bottombhole pressure from another well. It indicates that the flowing bottomhole pressure calculated by the equation is comparable to that by S1 and much more accurate than that by S2. Sensitivity analyses with the equation show that gas injection rate affects "static" pressure more than flowing pressure in the annular space in the tested data range. The new model indicates very high Equivalent Circulating Density (ECD) and low Equivalent Mud Weight (EMW) in shallow depth near surface. ECD and EMW versus depth in a 6.37"×3.5" annulus was generated for field applications. Introduction The drilling operations where the drilling fluid pressures in the borehole are intentionally maintained to be less than the pore pressure in the formation rock in the open-hole section is called Underbalanced drilling (UBD). The low borehole pressures are achieved by using lightened drilling fluids such as air, gas, foam, aerated oil, and aerated mud. The aerated mud is often the candidate for drilling permeable zones to handle significant formation fluid influx to the wellbore during UBD.1,2 Good designs are the key to the successful aerated mud drilling operations. Severe wellbore damages and failures can result from poor designs. Pressures in the annular space during drilling and after circulation break play vitally important rules in controlling borehole instability during aerated mud drilling, especially in inclined holes. As aerated mud is a compressible fluid, special care needs to be taken in hydraulics calculations. This is mainly because the frictional and hydrostatic pressure components influence each other through the pressure-dependent fluid density. Sophisticated numerical simulators are required to perform accurate computations. Both steady state flow and transient flow simulators are available in drilling industry for aerated mud drilling hydraulics calculations.3–7 Unfortunately, the results from these simulators are frequently conflicting8 due to assumptions that were made in mathematical formulations. It is, therefore, highly desirable to develop a simple and reliable hydraulics equation.
Micro-Flux Control: The Next Generation in Drilling Process
Drilling wells for oil/gas has been increasingly challenging with the companies moving towards difficult environments. The problems faced in these locations range from very narrow margin between the pore (or collapse) and fracture pressures, pore pressure uncertainty, to high pressure and high temperature wells. Wells drilled in these scenarios using the conventional drilling method often do not get to total depth (TD), and even so, drilling can be extremely risky, with several kick/loss situations. A new drilling method1 has been developed to overcome these problems, allowing a much safer condition, reducing the risks, and also permitting the wells to be drilled to TD much cheaper. Drilling is taken to the limit in a safe manner, extending each phase as much as possible, using the entire available mud weight window for that well. The method uses the new concept of micro-flux control, which is based on detecting a minimum loss or influx of fluids, and instantly adjusting the return flow and, consequently, the bottom-hole pressure to regain control of the well. The well is drilled closed at all times, and the return flow from the well is compared to the predicted and ideal one, allowing detecting the discrepancies in a very short time. This paper describes the basis of this new method and steps taken so far in the development. Field tests will be done very shortly, after the method has been tested in a simulated well condition. The use of this method allows wells to be drilled where today it has been impossible, extending today's technological limits way beyond the current boundary. Introduction Drilling wells for oil/gas is not a new activity, having started in the late 19th century. Despite being old, the drilling method has not evolved according to the development of new technology through the 20th century. It is essentially the same drilling method, using the hydrostatic pressure of the fluid inside the wellbore to control the pressures of the formations being drilled. There are several limitations of the traditional drilling method to drill wells in challenging environments. More severe operational restrictions, as well as environmental regulations, impose requirements that often make it uneconomical to drill wells in several locations around the world. Ultra-deepwater, high pressure and high temperature, and depleted reservoirs are just some. To overcome some of the problems caused by drilling traditionally in these environments, the industry developed some alternative techniques, such as underbalanced drilling (UBD), near balanced drilling (NBD), Low Head Drilling (LHD), and dual gradient drilling (DGD)2–15. All these alternatives were developed to solve specific problems and conditions caused by the traditional drilling method, such as loss circulation, formation damage, low rate of penetration, mainly caused by excessive overbalance pressure. Even though these alternatives are a step ahead from the conventional way of drilling wells, and bring significant improvement for the operation as a whole, they are far from being practical, economical, and being able to be used in a majority of the wells or even formation intervals. Therefore, they are still being used in a very restricted number of wells, in a niche market. On one side these alternatives can reduce the risk or make wells possible to be drilled, but, on the other side, they bring also severe side effects, as the equipments and techniques involved are more complex and require additional training and extra supervision, raising the risks of the drilling operation. The new drilling method described in this paper1 was developed combining the best of each available drilling alternative, including the conventional method, so that the disadvantages of each one can be reduced or eliminated. The main goal is to provide total flexibility to the driller to select which way the drilling operation should be conducted, with total safety and simplicity in terms of equipment and procedures. The paper describes initially the traditional drilling method, emphasizing the main points and disadvantages. Then, the paper briefly describes the most common drilling alternatives available and under development, to specifically address particular problems. The new drilling method is then described, followed by the advantages and benefits compared to the standard method and also compared to the various alternatives methods available.
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