| Popis: |
In the drive for excellent reservoir management practices and optimizing oil and gas production through efficient reservoir performance, the role of conventional coring cannot be overemphasized. Core data provides most representative and relevant data for the interpretation and understanding of the subsurface. Continuous coring techniques were deployed in the study area to obtain high quality cores for analysis and reservoir characterization of the productive intervals and development of appropriate enhanced oil recovery strategy. The reservoir cored is characterized as poorly consolidated sandstone reservoir comprising of a calcite roof, moderately sorted and very fine-grained, friable and laminated with shale. A thick column of shale intercalation separates the reservoir into upper and lower lobes. Generally, the formations were characterized as weak with the sand minimum stress of 7.5ppg to 11.0ppg and shale collapse pressure of 8.5ppg to 11.5ppg. Some of the challenges encountered in coring this reservoir includes but not limited to the following: Well collision issues, wellbore instability, depleted reservoirs, selection of effective coring bottom hole assembly and drilling fluid as well as rig space constraints. This paper presents how innovative techniques aided in mitigating the challenging drilling conditions. The systematic approach deployed comprises the use of gyro survey while drilling (GWD), anti-collision analysis, utilization of well-designed drilling fluid with low invasion properties that still maintain wellbore integrity and stability. A close monitoring of equivalent circulating density (ECD), use of fiberglass inner core barrel and low fluid invasion system that comprises core heads with special fluid pass ports to deviate flow away from already cut cores, and full core catcher system was employed. A detailed stratigraphic correlation with the offset wells to identify vital stratigraphic markers was carried out prior to coring. A 15 feet hole was drilled into the calcite roof of the reservoir with PDC bit and the hole cleaned prior to pulling out of hole with the drilling bottom hole assembly (BHA) to minimize the risk of core jamming. Cores were cut with special selected coring BHA using controlled coring parameters while plotting inverse rate of penetration (i-ROP). In addition, controlled trip out speed based on field experience, and proper core handling/ on-site core preservation was employed. The innovative approach resulted in achieving a recovery factor of more than 93 percent of high quality cores. |
