_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 226289, “Influence of EM Look-Ahead Service While Drilling on Geostopping Decisions in High-Pressure Gas Wells of Offshore China, ” by Shaopeng Zhu, Zi Rao, and Jialing Ma, CNOOC, et al. The paper has not been peer-reviewed. _ In offshore China, high-pressure (HP) gas reservoirs have become primary exploration and production targets, characterized by pressure coefficients reaching up to 2. 30 and extremely narrow safe-mud-weight windows. Globally, multiple successful field applications have demonstrated the effectiveness of electromagnetic look-ahead (EMLA) services for addressing these critical challenges while drilling by providing an accurate formation profile ahead of the bit to optimize geostopping efficiency. Introduction The Y Basin, a significant high-pressure, high-temperature (HP/HT) hydrocarbon-bearing basin in the South China Sea, has become a key focus for China’s oil and gas exploration. In Block L, the target reservoir exhibits typical HP/HT characteristics, with a static formation temperature of approximately 200°C and a formation pressure coefficient as high as 2. 30. The overlying shale layer has a lower pressure coefficient (1. 80 to 1. 90), creating a sharp contrast with the reservoir. This necessitates drilling in separate wellbore sections to isolate the shale from the reservoir. The transition between sections must be positioned as close as possible to the reservoir without penetrating it. Precise prediction of formation-pressure trends is critical for dynamically adjusting mud weight. Furthermore, ensuring drilling safety requires the advance prediction of formation characteristics and pressure variations ahead of the bit. While recent advancements in conventional methods have effectively improved look-ahead capabilities and guided HP/HT reservoir exploration in the Y Basin, these technologies fall short in fully addressing measurement resolution, prediction accuracy, and timeliness demands for these HP gas wells. Project Background and Objectives Block L is in the southern slope zone of the Y Basin. Drilled wells have encountered HP/HT gas reservoirs in the Miocene H Formation. The reservoir sequence comprises a thick shale cap at the top and five channel sand bodies in the middle-lower sections, transitioning from fine-grained to medium-grained sandstones with total thicknesses ranging from 10–30 m. Shaly interlayers separate these sand bodies. The sandstone reservoirs demonstrate an average porosity of 10%, classified as medium-low porosity and low permeability. Drillstem and wireline test data from drilled wells reveal reservoir static temperatures exceeding 200°C and formation pressures surpassing 100 MPa, with pressure coefficients reaching 2. 28. Tailored drilling-fluid densities must be applied to control wellbore stability effectively. Precise predrilling prediction of gas-reservoir positioning and potential pressure variations is critical. A 93/8-in. casing should be set within the shale layer, approximately 5 m above the HP gas-reservoir top to isolate formations with different pressure regimes. Correlation with the field’s limited number of offset wells reveals significant vertical variations in lithology and formation thickness with poor predictability, while the thickness of the overlying shale above the HP gas zones is highly uncertain. When the drill bit approaches the predicted formation, multiple geological circulations are needed to confirm the formation. This process not only reduces drilling efficiency but also introduces significant uncertainty and subsequent risks. If the intermediate casing shoe is set too far above the HP reservoir, although the risk of accidental penetration into the gas zone is reduced, the extended openhole shale section drilled with high mud weight may lead to wellbore instability, lost circulation, and reduced rate of penetration (ROP).
Chris Carpenter (Sun,) studied this question.