Geopressure Evaluation from seismic data and its application for exploratory wells in Mexico
Authors: Castillo J. Mancilla , Hernández E. Mendez , Santana-Fernández J. 
Published: October 2008

The quantitative evaluation of the overburden gradient, pore pressure and fracture gradient plays an important role in drilling successful wells mainly in gas basins and deepwater reservoirs. The ability to quantify geopressures, before and while drilling, is then a primary factor in reducing drilling costs.

The new strategy for exploring non-associated gas in Mexico has increased both the exploration and production activities. As a result, the exploratory wells have required more geopressure information before drilling in order to reduce the risks associated with subsurface formation pressures. To accomplish this objective, the Mexican oil government company, Pemex E&P, has addressed a strategy involving the Exploration and Drilling Departments to improve the understanding of geopressure to enable proper casing and mud program designs. In addition, an exploration understanding on trapping and sealing conditions has been addressed.

A good understanding of rock properties combined with the best geophysical practices of seismic pressure prediction have derived methodologies applicable to the process. An example summarizes the benefits involved in this multidisciplinary approach. Lessons learned and conclusions are included in this paper.

Introduction

Normal pressure is pore fluid pressure that equals the hydrostatic pressure of a column of formation water extending to the surface. Overpressure is an anomalous high pore fluid pressure in a formation, which is higher than normal pressure (Fig. 1). The phenomenon of overpressure is well documented in the technical literature. Several causes include geologic and hydraulic characteristics such as lithology changes, deformation, faulting and fluid expansion that are associated mainly with siliciclastic environments.

It has been estimated that abnormal geopressure costs the industry ﹩2-3 billion each year in blowouts and lost wells, "kicks", stuck pipe, unneeded casing strings or liners, etc. The industry has also reported that about 90% of deepwater well costs were due to troubles, mainly associated with geopressure. The ability to quantify these effects, before and after drilling, is then a primary factor in reducing these costs.

Accurate pressure prediction allows the proper casing and mud programs, avoiding down time in rig operation most often associated with stuck-pipe and lost circulation.

Traditionally, geopressure prediction has been based on basic data such as mud weights, repeat formation tests, leakoff tests, as well as sonic and resistivity logs, and very simple estimates of surface seismic velocity data. Recently, more robust seismic velocity analysis have opened new frontiers to geopressure prediction.

Pemex E&P has established the strategy of the multidisciplinary technological development to support the planning of the different responsible areas. In this context, the seismic prediction of geopressures is considered a strategic task for well drilling. Several approaches are being applied in Mexico to help to improve our understanding about predict geopressure variations on gas fields and frontier areas