Shale Picking Issues and Strategies: Key to Robust Real-Time Model Based Pore Pressure Prediction
Authors: D.Purkayastha Anjanava , Kumar Mrityunjay , Nair Pratap , Hansen Kirk S. , Couzens-Schultz Brent Alan 
Published: March 2015

Pore pressure prediction work is typically calibrated with pressure measurements from sands and log data from the adjoining shales. This is done because compaction effects on standard well logs are better preserved in shales than in sands. The prediction work assumes that sands and adjacent shales attain pressure equilibrium over geological time. To avoid lithologic effects on rock properties, calibration requires picking of similar shales from log data. Most commonly, clean end-member shales are picked. This paper addresses the issues encountered while picking shales for real-time pore pressure prediction (RTPPP) and the strategies to deal with these issues to confirm that changes in log measurements are related to variations in shale porosity and hence pore pressure.

Various shale picking issues that could impact RTPPP are: a) inconsistency in shale picking approach during model building phase and RT monitoring; b) lithology effects from presence of silt, carbonates, gas silts and organic rich shales; c) RT data issues related to logging while drilling (LWD) tool calibration and differences between phase shift and attenuation resistivity; and, d) anomalous log response like low gamma ray and low resistivity shales.

The strategies adopted to mitigate these potential issues are: a) use of consistent shale picking strategy in model building and RT monitoring phases; b) application of effective media models like Hashin-Shtrikman lower bound inversion and linear clay-volume modulation for silt correction and use of cross-plots to delineate the gas silts, etc., c) appropriate choice of resistivity tool and log curves to provide best measures of formation properties for each hole section and; d) validation of shale picks with well behavior, cuttings data and geological interpretation.

An optimized shale picking strategy integrated with well behaviour is imperative for predicting pore pressures within realistic bounds of uncertainty and for avoiding under/over prediction of pore pressures that in turn jeopardizes safe drilling.

Introduction

Knowledge of pressure regimes to be encountered in the subsurface and their uncertainty prior to well design and whilst drilling is critical and pivotal for successful exploration and development. Overpressures in the subsurface can cause major problems from the perspective of cost-effective well design and safety. A partial list of issues affected by pore pressure includes (modified from Gutierrez et al., 2006):Safety in drilling and cost: Rig selection; non-productive time (NPT) associated with the trouble events like well kicks and blowouts; lost wells; wellbore stability problems; mud expense and mud loss; stuck pipe; formation damage; extra casing runs.Environmental risks: Spills associated with blow out; seafloor breaching; aquifer contamination; etc.Prospectivity and trap analysis: Evaluation of hydrocarbon retention and column height; sealing/non-sealing faults; top seal capacity; aquifer continuity/pressure support during production; volumetrics and economics.Impact on rock and fluid properties: Reservoir quality; sediment and fluid acoustic properties and quantitative seismic interpretation.