Drillability Assessment in Deepwater Exploration
Authors: Albertin Martin L. , Petmecky Stephan , Jay Charley , Vinson Pinky 
Published: October 2008

Velocities derived from modern 3D seismic datasets are useful for evaluating both pressure and drillability variations on a variety of scales. Drillability assessment, which accounts for vertical variability in the pore-fracture pressure window, is applicable early in the exploration cycle as an indicator of drilling complexity and cost. At the regional scale, where drillability is evaluated on a relative basis, errors which plague velocity based pressure predictions are relatively unimportant.

At the prospect and wellbore scales, pore and fracture pressure estimates directly impact wellbore planning and drilling performance. Though pressure prediction accuracy is extremely important, seismic velocities, regardless of the method through which they were derived, do not provide sufficient resolution and accuracy to be used independently for well bore planning. A successful drillability assessment workflow at the prospect scale entails integration of analog well data, high resolution velocity analysis, detailed 3D mapping of sand distribution from mudline to TD, centroid modeling, and 2D / 3D basin modeling.

Introduction

Accuracy and resolution of velocity data derived from modern seismic surveys has improved over the past decade because of advances in seismic acquisition and processing technology, and emphasis on derived velocity data as a valuable processing end product. With higher quality input data, and evolutionary improvements in the empirical models used to convert velocity to pressure, pressure predictions are more accurate now than ever. But despite improvements in pressure prediction technology and methodology, enormous drilling resources continue to be spent on mitigating pressure related problems.

The substantial cost savings that improved drilling performance promises, particularly in the expensive deepwater drilling environment, is a strong driver behind efforts to improve pressure prediction accuracy, and to extract more information from existing seismic datasets. Another motivation for improving not only the way we derive pressure, but also the end products of our pressure prediction workflows, is the recognition that understanding relative changes in pressure, effective stress, and drillability across exploration trends is valuable early in the exploration cycle.

We make a distinction between pressure prediction and drillability assessment. In pressure prediction workflows for wellbore planning, we provide the most accurate estimates of formation pore pressure and fracture strength possible. This is accomplished by integrating multiple independent pressure prediction techniques, so that the weaknesses in any particular method are compensated for by strengths in the others. Detailed pressure prediction efforts usually begin late in the exploration cycle, after drillable prospects have been identified and engineering efforts are underway to design a well which will successfully test the exploration objectives. In drillability assessment, we de-emphasize absolute accuracy in pressure estimates, and focus more on relative changes in pressure across exploration trends. In addition to pore and fracture pressure, we use derivative attributes which are more discriminating in identifying areas where drilling might be problematic, and seal capacity might be an issue. Drillability assessment, because it emphasizes relative changes in pressure attributes, is easily done much earlier in the exploration cycle and does not necessarily require input from alternative pressure prediction methods.