Oil-base mud (OBM) is an increasingly popular drilling fluid used to protect the wellbore stability in horizontal wells drilled in China targeting shale gas or oil. In these reservoirs, an in-depth understanding of fracture attributes is critical for stimulation design and well placement optimization, but the resolution and coverage of traditional imaging technology for OBM limits the extent to which this objective may be achieved. A microresistivity imager achieving resolution and coverage similar to established technology for conductive water-base mud (WBM), fully capable to distinguish fractures and formation structural features in OBM, has been a much needed technology development to provide critical insight required for shale oil exploration.

The paper presents a case study from the Bohai Bay basin in Eastern China, where oil shale formed in the faulted depression lacustrine sub-basins of the Lower Oligocene. One horizontal exploration shale oil well was drilled with OBM for reservoir evaluation. Only a gamma ray log and new, high-definition microresistivity image logs were acquired due to difficult operating conditions, which made it a great challenge to interpret reservoir properties and optimize stimulation design. Fortunately, two nearby vertical wells were drilled and evaluated in detail prior to design of the horizontal well, making use of geochemical, advanced acoustic, and microresistivity image logs to optimize placement of the lateral, thus providing a reference for estimation of the reservoir properties.

The results showed that the new, high-definition resistivity imaging tool gives detailed images suitable for full fracture and structure characterization, and that such high-quality images are feasible even in wells drilled with OBM. Interpretation of high-definition microresistivity images to build a near-well structural model provided a framework for correlation of fracture, reservoir, and mechanical properties from the vertical wells to the horizontal well. This provided a clear picture as to how the lateral well trajectory progressed stratigraphically within the shale section, as well as the distribution of the fractures, reservoir and non-reservoir rocks, total organic carbon (TOC) and minimum geo-stress along the well trajectory. Subsequently, this rich model provided the needed critical inputs to decide how and where to complete the well with an optimized stimulation design.

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