Lessons Learnt from a Successful Sampling While Drilling Campaign Delivering Formation Oil Samples and Saturation Pressure Measurements in High H2S Carbonates

Abstract

As island development strategies gain focus for capitalizing deep offshore assets, limitations like fixed slot location bring about the need for drilling extended reach (ERD) wells with multiple drain holes and complex well geometry to maximize the reservoir coverage for increased production. Pressure testing and reservoir fluid sampling operations require long stationary time and pose a risk of differential sticking. Deploying a pressure testing and fluid sampling tool into the drilling bottom-hole assembly (BHA) helps in maintaining well control through continuous circulation and providing measures to retrieve the tool by rotation and jarring in case of pipe sticking. This paper presents the successful deployment of sampling while drilling tools in three ERD wells drilled using water based and oil based muds to acquire representative formation oil samples from a high H2S carbonate reservoir. The formation oil samples were collected immediately after drilling the well to the target depth for limiting the invasion to collect clean samples in shorter pump-out volume and time. After securing the samples, a phase separation test was performed by fluid expansion in a closed chamber to measure the saturation pressure of the oil. A 30-min long pressure build up was also performed for pressure transient analysis to estimate permeability. Formation fluid samples were collected, while pulling out the drilling BHA, within 12-48 hours of drilling the well by pumping out 100-170 liters of fluid from the formation in 4-6 hours. During clean up, absorbance spectroscopy identifies the fluid phases – gas, oil and water. Prominent trends observed in compressibility, mobility, sound slowness and refractive index measurements add confidence to the fluid identification and provide accurate contamination measurements. Single-phase tanks charged with nitrogen were used to assure quality samples for PVT analysis. The sample tanks are made of MP35N alloy and the flow lines are made of titanium that are both H2S resistant and non-scavenging materials and hence, a separate coat of non-scavenging material was not required. In highly deviated wells, sampling while drilling technology can close the gaps of the conventional wireline operation on pipe conveyed logging in addition to saving 5-days of rig time by eliminating the need for conditioning trips, a dedicated run for pressure testing and sampling and minimizing the risk of stuck pipe and well control incidents The results from downhole fluid analysis and PVT lab are compared in this paper. Going forward, this technology can eliminate the requirement of a pilot hole for pressure testing & sampling by enabling sampling in complex well geometries in landing sections and ERD wells. The paper concludes with discussions on suggested improvements in the tool design and capability and recommendations on best practices to align with the lessons learnt in this sampling while drilling campaign.