It has become extremely important in recent years to do a better job in well planning to ensure that well cost estimates and drilling programs are as complete and accurate as possible. Comprehensive well programs and realistic estimates are necessary to determine which drilling prospects are most attractive considering current upstream economics.
Pore pressure analysis is the initial, and in most cases, the most important step in developing an accurate, complete well program. Casing setting depths, hole sizes, number of casing strings, mud programs, and hydraulics are examples of items in a well program which are influenced by the pore pressure profile determined for a well. Pore pressure analysis is especially critical for exploratory and delineation wells but until recently our-predictive techniques were somewhat inadequate.
This paper will present the application of seismic correlative and seismic velocity (ITT) techniques to predict pore pressures on a North Sea exploratory well prior to drilling. Actual mud weights used on the prospect are compared to the predicted pressures.
The steps used to predict pore pressures will be discussed including sonic log pore pressure analysis correlated to the prospect, development of a regional pore pressure overlay for interval transit times, and analysis of ITT information developed from seismic shot points at the prospect and an offset well. Results from this interpretation of seismic information will be compared to the actual pressures determined after drilling the well to emphasize the effectiveness in using these techniques for pore pressure prediction.
The pore pressure profile for any well is essential for development of an accurate, comprehensive well program and cost estimate, especially in geopressured drilling environments. Pore pressure prediction is therefore extremely important for delineation and exploratory wells where control well information may be minimal or nonexistent.
The relationship of pore pressure to predicted fracture gradients for a given area determines casing setting depths. For this reason, reliable pore pressure prediction for a prospect well must be the first step in preparation of a drilling program. It follows that the number of casing strings and predicted pore pressures dictate projected hole sizes, casing and drill string design, mud programs, planned hydraulics programs, e.t.c., which are ultimately incorporated in the drilling program for a prospect.
Historically we have had to rely solely on offset well information to predict pore pressures for prospect wells. Pore pressures have often been determined based on measured or calculated pressures in the various formations penetrated in an offset and assumed to be the same for those formations expected in a proposed well. Although this may be the case, most of the time this prediction will be wrong since formations are not always penetrated at the same depth between two wells, the wells may not be hydraulically related, and the presence or lack of hydrocarbons will influence the predicted pressures.
We have identified steps necessary to predict pore pressures in most wells, including exploratory and delineation wells with little control, which have proven reliable and have significantly enhanced well planning and operational control while drilling.