The CD Carbonate in X-Field, East Java Basin has multiple reservoir targets with varying flow units, predominantly because of secondary porosity development from karst processes. Each of these reservoirs are relatively thin and the continuity of the karst within the reservoir is random. The amount of producible hydrocarbon will rest on the length of the drain section along the reservoir and the amount of karst intersected by the trajectory. Thus, maintaining the trajectory inside the carbonate reservoir while targeting the karst position is important for productivity. To maintain the production in the field, a new platform was built to the southeast of the first platform, aiming for a new development area. Five horizontal wells were planned as part of the second development phase. The first horizontal well is the most challenging one because of the high uncertainty of the structural dips along the lateral length triggered by the low seismic resolution and the limited nearby offset wells for control points. Moreover, the offset wells show inconsistent log properties that complicate the correlation to land into the targeted karst level within the reservoir. Initially, a pilot hole was planned to reduce the landing uncertainties; however, it was being challenged for cost efficiency. Therefore, a well placement strategy was proposed instead, including landing and geosteering using a new logging-while-drilling (LWD) combination of advanced high-definition reservoir mapping technology, high-resolution laterolog borehole imaging technology, and a multifunction LWD tool. In this paper we demonstrate the complete preparation of the well placement project, strategy, and evaluation using this new LWD combination for better interpretation of the reservoir. The deeper reading and higher resolution of the new reservoir mapping technology have permitted continuous mapping of the target reservoir, which typically has 35- to 50-ft thickness, to reduce the structural uncertainties from seismic. For the first time, it successfully revealed the karst network within the reservoir with greater details, identified by a blue-vein color spectrum of the resistivity inversion caused by seawater invasion or clay-filled karst. This high-definition karst mapping has helped to land the well precisely at the target karst sweet spot, improving the understanding of the karst characters along the trajectory, and providing higher confidence in the real-time geosteering decision. The high-resolution borehole image revealed the carbonate rock texture and karst/vugs appearance on a smaller scale, which was used to analyze the secondary porosity distribution and contribution along the trajectory using a quantitative image-based porosity analysis method. By integrating the high-definition reservoir mapping inversion interpretation and porosity analysis from a high-resolution borehole image, we were able to bridge the gap from seismic to reservoir scale, and finally to link the karst scale down to the vug pore sizes, for a better geological understanding and an improved geosteering strategy in the field.
After more than 20 years of intensive production in XiJiang oil field, a sand group that consists of multiple reservoirs with a thin oil column of less than 4 m and with strong bottom water drive has been revisited. Some assessments were made previously, including drilling horizontal wells with average recovery factor or only 2.3%. In early 2013, new horizontal well drilling was initiated to reassess these reservoirs with a different approach. The objectives were to optimize the standoff between the lateral and the oil/water contact, which is very critical to well performance, by placing the lateral as close as possible to the reservoir top and to evenly regulate the downhole flow across the draining lateral. The program was successful in reassessing the development reservoirs in the mature oil field that had previously been considered to be uneconomic through a reentry horizontal well drilling program and the implementation of the best practices in operations. The operational practices included the precise landing and lateral placement of horizontal wells and the use of inflow control devices (ICD) for completion. Logging-while-drilling (LWD) bed-boundary mapping, with the ability to map multiple key boundaries, including fluid contact for precise well placement, was integrated with multifunction formation evaluation LWD, which provides real-time formation evaluation to optimize the ICD design for completion. Outstanding outcomes have been observed upon the completion of the well: Producing with 1,600 BOPD with 5% water cutAchieving 56% higher cumulative incremental oil than the best previous horizontal wellExceeding the economic level threshold to develop Comparison of new and the past well performance support the acquisition of the new field data. The successful approach, best practices, and valuable lessons learned provided a breakthrough in developing the previously unexploited reservoir.
An oil & gas operator in XiJiang block offshore South China has shifted the development of highly mature oil fields that have been producing for 17 years toward the remaining undeveloped marginal reservoirs using a horizontal well drilling program. The main targets of the new development campaign are the remaining thin oil column reserve in attic locations of the reservoirs as well as the unproduced thin laminated reservoirs in the area. Upon understanding the uncertainties and challenges associated with drilling horizontal wells in these complex reservoirs, an innovative drilling approach was initiated for accurate horizontal placement in thin sands, channel sands, and thin oil column reservoirs with strong bottom-water. The approach includes the integration of an advance multifunction formation evaluation Logging-While-Drilling (LWD) tool that provides real-time formation evaluation and structural interpretation along with a bed boundary mapper LWD tool with the ability to map multiple key boundaries that are the prerequisite parameters that must be identified during the execution stage and which include the water contact and top and bottom of the reservoir structure simultaneously in distance. Outstanding outcomes have been observed by implementing this new approach in the complex target reservoirs. The approach was applied to multiple challenging wells, each of which has produced from 2,000 to 6,000 BOPD with very low water cut, far exceeding the set production goals of 1,500 to 2,000 BOPD. These are very promising development economics in the oil fields that have 90% to 95% water cut on average. The successful implementation of the new development strategy in highly mature oil fields will lead to sustained and extended production, increasing the ultimate recovery and well economics. The approach provides an example of using integrated technology solutions to overcome the challenges in complex target reservoirs.
Carbonate reservoirs in East Java basin had been known for its challenging drilling conditions due to the overlying shale instability and heterogeneity. The primary carbonate formation in the field has multiple reservoir targets with different flow units, which are dominantly due to secondary porosity resulting from karst processes. These reservoirs are relatively thin with net thickness ranging from 10 to 40 ft. Thus, the amount of producible hydrocarbon will depend on the length of the drain section along the reservoir. Horizontal well was selected by the oil operator in order to have economical production rate from each of these carbonate reservoirs. However, there are high uncertainties on the structural dips along the planned lateral length due to limited numbers of offset wells and low seismic resolution. Moreover, karst has irregular shapes and sizes that can pose additional challenges such as directional drilling control, loss circulation, and deep mud invasion that affect shallow measurements. Technology with deep directional measurements that is capable to map the reservoir boundaries and structural dipping beyond the karst zone, is therefore required to achieve the horizontal length objective. This paper highlights a successful horizontal well placement in a karstified carbonate layer utilizing the new LWD reservoir mapping technology, in conjunction with the other LWD measurements that provide the necessary petrophysical information for thorough reservoir evaluation. This new deep directional electromagnetic technology extends the depth of investigation to 100 ft or more from the wellbore and resolves multiple layers with contrasting resistivity, for reservoir-scale imaging capability. As a result, the lateral drain section was maintained successfully in the target reservoir with total net length of ~2600-ft. This interval consists of 45% length in karst or secondary porosity and 55% in the matrix-dominant porosity. Multiple layers with depth of investigation up to 50 ft were mapped in the inversion result from Reservoir Mapping-While-Drilling technology, where top and bottom reservoir boundaries were continuously mapped during drilling. In addition, reservoir mapping application in this well revealed the various heterogeneity natures in the reservoir such as lateral changes due to karst system diagenesis process and a sinkhole feature. These informations are useful for improved reservoir evaluation and delineation. The integration between reservoir-scale imaging and the geological concept of the field not only has the capability to map the reservoir boundaries and structural dips around the trajectory, but also led to optimum steering decision. Following the success, similar method and technology will be used in the subsequent horizontal wells in this field.
Dual-lateral horizontal wells have been the strategy applied in Changbei field to maximize economic development of a tight gas sandstone reservoir. A motor and measurement-while-drilling (MWD) bottomhole assembly (BHA) is normally used to drill 2-km dual-lateral wells within the quartz arenite (QA) sand reservoir that has an average thickness of 15 m. After completing most of the drilling in the thicker axial region of the channel belt, the field development has now shifted toward the channel margin boundary that has more straigraphic complexity and much thinner reservoir. A pilot hole, drilled to provide better understanding of the reservoir thickness near the channel boundary, has revealed that the reservoir was about only 1.5 m in thickness. The application of motor and MWD BHA would have not been adequate to drill and place the well within the very thin reservoir. Therefore, the combination of RSS and the high-resolution resistivity image LWD BHA were applied to replace the motor and MWD BHA. This paper features the successful approach taken using an integrated application of RSS, optimized bit, high-resolution resistivity image LWD, and dip determination well placement technique to overcome the subsurface challenges and to improve the drilling efficiency. RSS and LWD technologies were deployed to provide real-time, full-bore, high-resolution resistivity and gamma ray images to evaluate the structural dips, image stratigraphic events, and provide better trajectory control with near-bit survey capability. The field real-time data will be discussed. This integrated application has helped to place the well optimally along the thin target reservoir, thus optimizing the reserve recovery despite the large discrepancy of actual reservoir profiles relative to the original predrill model. The authors also share the successful approach, best practices, and valuable lessons learned that have provided the breakthrough in developing the tight gas reservoir under such complex geological uncertainties and in a very hard, abrasive formation. Introduction Changbei gas field is located in ShanXi province, northcentral China. It resides on the edge of the Mauwusu desert in the Ordos basin, one of the largest sedimentary basins in China. It is also the first tight gas field in the Ordos basin to successfully use horizontal dual-lateral wells as the basic development concept. Dual-lateral horizontal technology was introduced to achieve high production rate without applying fracturing technology. The horizontal well can penetrate several potential baffles in the reservoir, enlarge the reservoir exposure length, and extend the drainage area. The main Upper Paleozoic clastics reservoirs in Changbei field occur 2800 to 3000 m below the surface. The target gas reservoir is located in the Lower Permian Shanxi formation (P1S2 QA). The P1S2 QA interval is defined in core and calibrated to logs as a low-gamma quartz arenite main reservoir sand. The main P1S2 QA reservoir has an average gross thickness of approximately 15 m (1.5 to 39 m range) and displays a regional SW-directed dip of ~1°. Drive mechanism is depletion drive with dry gas.
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