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In overbalanced drillng (OBD), a mud weight is selected that provides a hydrostatic pressure of 200 to 1,000 psi above the reservoir pressure. In UBD, we select a fluid that provides a hydrostatic pressure of around 200 psi below the initial reservoir pressure. This provides a good starting point for the selection of a fluid system. During the feasibility study, this drawdown is normally further refined, depending on the expected reservoir inflow and other drilling parameters. This first look provides an indication if the fluid should be foam or gasified or if the well is drilling with a single-phase fluid (Figure 1).
Abstract Major challenges faced by today's industry in drilling fractured carbonate reservoirs are formation damage and potential drilling problems such as fluid loss and differential sticking. To drill such formations, underbalanced drilling (UBD) technology has been used as it can intentionally keep the bottomhole pressure (BHP) lower than the pore pressure (PP), which will eliminate the near-wellbore damage, and mitigate drilling problems like fluid losses. This technology was also used onshore in Thailand, mostly for development wells with low-pressure reservoirs. Well-A, located in the fractured carbonate Pha Nok Khao (PNK) formation of northeast Thailand is an exploration well with expected reservoir pressure around 6,000 psi (13 ppg in equivalent density), whose operator was determined to utilize UBD for the production section in order to minimize formation damage and maximize and test productivity. The higher reservoir pressure leads to higher operational risks for UBD. While drilling, single-phase water based mud (WBM) was used to keep proper drawdown for this section. In order to have safe and efficient tripping operations, tandem downhole deployment valves (DDV) were utilized to provide ultra-reliable isolation of the wellbore for all the trips during drilling, logging and completion, which eliminated the need to snub or kill the well—any of which would lead to potential formation damage and safety issues. After completion, isochronal well testing was conducted using the same UBD package and later, in order to obtain more data to determine whether the operator could commercially develop the field. After that, a long-term well-test operation was conducted by continuously flaring the well at variable rates for 15 days using the same package. As the well was in an underbalanced condition through all the operation phases, no formation damage, fluid losses and NPT happened during the whole operation. The use of same UBD package from drilling till well testing for the full operation cycle also minimized costs and rig time. This paper details the UBD approaches used to minimize formation damage and evaluate reservoir productivity for the Well-A in the PNK formation of northeast Thailand.
Abstract Foam is commonly used for UBD (underbalanced drilling) because of its low variable densities; which make adjustment of its densities possible in order to keep control of the circulating bottomhole pressures, and because of its high effective viscosities; which give a superior cuttings lifting and transport ability. Foam is also used to transport formation fluids that enter the borehole while underbalance-drilling. This paper is intended to propose an improved computer program in the VISUAL BASIC language that can be used to better simulate the foam hydraulics for vertical wells. The developed model considers foam as a Non-Newtonian power-law fluid in spite of the dispute that still persists among the researchers. Results of the proposed model revealed that the foam flow is affected by the injection pressures, injection flow rates, bottomhole temperatures, ROP (Rate of Penetration), cutting sizes and densities, fluid influxes and the surface back-pressures. The model also proposes a try-and-error procedure to initially determine the best selections of the injection pressures and the injection rates for both liquid and gas. The model evaluation was tested by running the program on two field wells drilled underbalanced with foam in the Middle East with an absolute average error of less than 2.6 % for the first well and with an absolute average error of less than 10.9 % for the second well.
Liu, Wei (China National Petroleum Corp.) | Shi, Lin (China National Petroleum Corp.) | Zhou, Yingcao (China National Petroleum Corp.) | Wang, Ying (China National Petroleum Corp.) | Jiang, Hongwei (China National Petroleum Corp.)
Abstract In the complex geological condition, drilling operation often encounters the stratum whose fracturing pressure and formation pressure is very near. It is easy to cause drilling complex problems such as well kick and well loss. In order to solve the problems, a pressure control drilling system (PCDS) was developed, including automatic manifold system, back-pump system and auto-control system, which can control the pressure distribution in the entire borehole annular precisely through closed-loop automatic control system. The kernel of PCDS is auto-control system which has three significant characteristic, including an advanced real-time hydraulic method, simple and effective control system, self-adaptive control strategies. The auto-control system configuration composes of field-bus device, controller and workstation which form three levels. Level? is measure and on-site diagnosis level. Level? is monitor and control level. Level ? is optimized control level. From 2011 to now, the PCDS has successfully finished six field services in China which can provides additional flow-rate and back pressure to compensate for the reduction or increment of bottom hole pressure while making a connection, reduction in rig pump rate, change in mud weight or drill pipe movement and so on. The underbalanced-pressure control technology is also developed which is different from the conventional PCDS technology because the controlled bottom-hole pressure is lower than pore pressure of formation and the gas or fluid can be allowed to come out from formation at a controlled rate which is benefit to protect reservoir and improve ROP. The applications prove that the PCDS equipment can control the bottom-hole pressure in an exact range and the precise pressure control technology of underbalanced drilling is feasible. So the PCDS equipment can easily adapt to do over-balanced, near-balanced and under-balanced drilling operation and will be more successful in the oil or gas well with safety and fast drilling in the future.
Abstract This paper discusses how the challenge to improve production from a depletedoil field in Algeria was approached using an advanced Coiled Tubing DrillingSystem. The paper covers the planning and implementation of the project anddiscusses lessons learned, results and conclusions. Introduction The Rhourde El Baguel (REB) oilfield has been on production since the early1960's. Only 20% of reserves (estimated at 2 billion OOIP) have been recoveredto date. Approximately 84 wells have been drilled in the field so far, mostlyvertical. In the reservoir, matrix properties vary from moderate to very poor, with permeabilies in the range 0.02–10 Md. It can be generally classified as atype II Fractured Reservoir. As production is dominated by flow fromsubvertical natural fractures, the challenge was to drill horizontally andintersect regularly spaced conductive fracture swarms (see Fig. 11). Thereservoir is highly depleted, having a pressure of 1800 psi at 2900m TVD (EMW3.64 ppg) - the original reservoir pressure at discovery was approximately 5600psi. The Cambrian sandstone is extremely hard and abrasive: with compressivestrengths ranging between 15,000 psi up to 35,000 psi (see Fig. 1). There isalso a high degree of stress anisotropy, and borehole breakout was a majorconcern for both the drilling and production modes. This paper describes how the project was evolved, planned and implemented tosuccessfully meet these technical challenges by sidetrack drilling, underbalanced out of existing well bores with the completion in place whilealso meeting stringent heath, safety and environmental challenges.