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In designing a UBD circulation system, the bottomhole pressure must be maintained below the reservoir pressure. The surface separation system must have sufficient capacity to handle the flow rates and pressures expected while drilling. The surface separation system must be capable of handling sudden productivity increases from the well from fractures or flush zones and retain the ability to "choke" back production if well outflow is more than what can be handled safely by the surface separation equipment. The separation system must also be able to work within the design parameters of the well. The BHP must be less than the static reservoir pressure under static and dynamic conditions to enable reservoir fluid inflow into the wellbore.
Abstract The success of recent applications in underbalanced drilling (UBD) and managed pressure drilling (MPD) has accelerated the development of technology in order to optimize drilling operations. The increased number of depleted reservoirs and the necessity for reducing formation damage has also increased the need to apply UBD/MPD to such candidate fields. Several methods used the latest mechanistic multiphase flow models in order to predict bottomhole circulation pressure when performing UBD/MPD operations. A new model is developed that utilizes the latest mechanistic multiphase flow models; the developed model calculates the bottomhole circulation pressure as a function of surface injection rates, choke pressure and time. The developed model can be used in designing and optimizing UBD/MPD operations in terms of determining the correct injection rate and/or choke pressure. In addition, the developed model is used to utilize the reservoir energy to attain correct bottomhole conditions. The developed model in addition to utilizing the latest mechanistic models also reduce the error in calculating the bottom hole pressure by incorporating an algorithm in which the injection rates are calculated in-situ rather than assuming constant injection rates. The model is validated against data from literature and against a commercial simulator. Results show that the developed algorithm has increased the accuracy in predicting bottomhole pressure by incorporating the changes in new gas and liquid injection rates.
Abstract Gas encountered in deep carbonate (Khuff) and deep sandstone (Jauf and Unayzah) formations require significant planning and impelemntation of high-end technology for optimized exploitation. The conventional drilling methods for vertical, horizontal, and maximum reservoir contact (MRC) have been used extensively to produce the reserves. The need to avoid formation damage and minimize differential sticking has become important to maintain the well integrity and rates, particularly in reservoirs that are tight and encountering pressure depletion with time. Underbalanced Coiled Tubing Drilling (UBCTD) technology is a breakthrough in developing mature and depleted reservoirs. With UBCTD, workover operations, such as drilling laterals in very thin reservoir sections and controlling their trajectories with flexibility to steer into net pay sections, are achieved with more accuracy. Also, the high operational costs with rig time, the need of inducing hydraulic fractures and drill-in mud invasion caused by conventional drilling are avoided. In addition, the technology plays a major role in eliminating pressure differential sticking while drilling across multilayered reservoirs that exhibit layering depletion. The technology was introduced after conducting a pilot project with six wells where post-UBCTD production performance were analyzed and exceeded expectations. In some cases, UBCTD provided as much as a threefold increase in initial well productivity and achieved a higher, long-term sustained rate compared to conventional workover operations conducted in similar reservoirs. Due to the ability to continuously monitor well performance during drilling and the higher flexibility of coiled tubing (CT) operations, changes in drilling direction and inclination could be easily made to maintain the drill bit in layers of interest and enhancing reservoir contact in the desired sections. This paper presents two actual examples from carbonate and sandstone reservoirs where UBCTD has been successfully deployed. The post-workover pressure and production performances and comparison with results obtained from conventional drilling are presented to illustrate the improvement in gas rate. The paper also presents pressure transient tests conducted after workover operations indicating low or zero skin damage and high permeability-thickness (kh) as a result of effective minimization of wellbore damage from drill-in mud and achieving higher net-pay contact.
Abstract Underbalanced drilling (UBD) is a method where equivalent circulating density (ECD) is kept less than the effective pore pressure of the zone being drilled by 100 psi or more. Drilling in an underbalanced state provides a unique opportunity to gather real-time virgin reservoir data that would never be observed again after typical damaging effects from conventional overbalanced drilling. This condition allows formation fluid to flow into the wellbore during the drilling process, helping prevent drilling mud and associated contaminants from penetrating the producing formation, which protects the producing zone and minimizes the potential for formation damage. Proper instrumentation and drilling procedures allow acquisition of data that is then interpreted and analyzed to extract information about the reservoir. UBD benefits include: Increased rates of penetration by up to 50% Increased value by producing hydrocarbons while drilling Increased production rates mean quicker payouts Reduced formation damage Reduced lost circulation costs Reduced potential risks of differentially stuck pipe An operator in India was truly the first to implement UBD technology within depleted reservoir sections of identified wells in an offshore asset. Three wells were planned as a part of the project. Project objectives, feasibility studies to determine applicability of the UBD technology to the selected field, an operational summary, and results of UBD operations on the first well are discussed. Objectives for implementing UBD technology within the identified field included helping prevent reservoir damage, thereby enhancing oil production and recoverable reserves from the wells, and helping improve drilling performance within the reservoir section by eliminating mud losses, hence increasing lateral length and reservoir contact.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 178089, “Case History: HP/HT Underbalanced Drillstem Testing, Deepwater KG Offshore,” by Mahesh Sarode and Milind Khati, Halliburton, prepared for the 2015 SPE Oil and Gas India Conference and Exhibition, Mumbai, 24–26 November. The paper has not been peer reviewed.
This paper describes the method developed to achieve underbalanced drillstem testing (DST) in a deepwater field offshore India. DST tools rated to 450°F and 15,000-psi differential pressure were configured to maintain integrity and successfully evaluate the well potential. By use of a multicycle DST tool string, the reservoir potential was tested safely and effectively with an underbalanced test fluid in two deepwater wells, achieving the desired test objectives.
An operator in India needed to conduct high-pressure/high-temperature (HP/HT) drillstem testing on a deep-water well with low-density clear brine in the annulus. Performing DST was necessary to assess hydrocarbon potential in the deep water block for further field- development plans. The DST tools performed satisfactorily for approximately 18 days during the test on a semi-submersible rig.
The well was completed in a 6-in. hole, which resulted in a 5-in. casing program, thereby creating a significant challenge. The retrievable packer suitable for the 7-in. casing was set above the 5-in.-liner-hanger top, resulting in a tail pipe of approximately 350 m. Well killing at the end of the DST with such a long tail pipe was a challenge.The method of operating DST tools in both underbalanced brine and kill-weight mud was developed along with a meticulously designed well-test program and a well-kill procedure with a long tail pipe below the 7-in. retrievable packer. Before lowering the DST tools, a well- integrity test was performed by conducting an inflow test for a 5-in.-liner shoe, a 5-in.-liner-hanger top, and a 7-in.-liner-hanger top. Hermetic tests were per-formed to ensure that casings sustained the required annulus pressure for operation of the DST tools.