Focused Sampling – The Way to go in Low Permeability Reservoirs

Abstract

To ascertain reserves and future development of low permeability formations drilled with oil-base mud (OBM) in a highly deviated offshore well, low contamination hydrocarbon samples were required. With limited sampling time allowed, due to historical stuck-tools cases within the same environment, different sampling technologies were appraised. Focused sampling was selected based on detailed near wellbore formation simulation. The sampling technique was successfully deployed, with a single tool string descent, in a complex 3D 6" slim wellbore (S-profile with 51 degrees inclination followed by a tangent, then a J-profile followed by another tangent). Different sampling techniques were attempted earlier to sample in the low permeability formation using various single probe sizes and inflatables, but none were successful to prove hydrocarbons, mainly due to the allowable sampling window, and tool sticking indicators that prevented longer cleanup duration. A rigorous pre-job near wellbore simulation modeling was performed to envisage varied scenarios and be prepared for contingencies. Sensitivities were run for thickness, kv/kh, sampling depth and permeability. Consequently, elongated focused probe sampling technique and straddle packer were selected. The inflatable straddle packer was kept as a contingency. Focused sampling has undergone dramatic changes over time by combining different probe sizes / types. This type of sampling technology uses the outer flow produced at a guard inlet to prevent mud filtrate from migrating into the sample inlet. Three sampling points were selected based on the pressure data. Despite the long tool string and harsh environment, samples were successfully collected. When compared to the previously drilled well within the same reservoirs, this is considered as a major achievement to secure samples which eventually proved oil and secured a license. Cleanup time varied between 2.0 hours and 3.5 hours, with 96 to 142 liters pumped volume. The inflatable straddle packer was not used. This saved more than 12 hours of rig time and secured risk-free samples with a smooth operation. Laboratory Analysis of the hydrocarbon collected samples indicated OBM filtrate contamination ranges between 3.0% and 5.0%. Pre-job near wellbore simulation proved to be a powerful tool to study sensitivities and extremes at sand face, which helped deploy the optimal sampling solution in a time constrained and mechanically challenging scenario. The post-job near wellbore simulation helped to fine tune some of the reservoir parameters such as thickness, which would be helpful for future applications. This was the first deployment of extended range probe focused sampling (elongated focused probe sampling) in the low permeability reservoirs in Malaysia and Petronas, and the same was successful. The risk of using inflatables in slim hole was avoided. The high-quality sampling was achieved in one tool string descent, allowing timely well completion and met the first oil timeline.