Pore and fracture pressure trends were analyzed to provide recommendations and support drilling of deep wells in non-consolidated formations of the Gulf-of-Mexico fields (Matagorda Island and East Cameroon). The main objectives were to predict pore pressure and fracture gradient, analyze compartmentalization, aid to well design and control drilling in the overpressured sections. We were particularly interested in understanding the role of diagenetic changes and tectonic activities (faulting) that control (enhance/damage) the sealing properties of shales. Faulting is an important mechanism in forming of the overpressured zones, but on the other hand it can partially destroy shale sealing properties. These two different events were observed in the Gulf-of-Mexico area and thoroughly documented. Assessment of both examples allows defining an influence of particular fault and predicting formation pressure with more confidence.
INTRODUCTION AND BACKGROUND
The scope of this paper is to analyze the results of drilling in overpressured zones, plus incorporate previous data and analyses of wells of Matagorda Island and East Cameroon fields and based on that provide recommendations tbr future drilling in the Gulf-of-Mexico area. Our main objective was to assess the influence of various faults on shaly sealing properties and well bore stability. Material for this study was collected over past several years when an effort was made to predict pressure compartmentalization and position of casing points in eight wells drilled.
The features we have to account for, while drilling into a overpressured zones are the decrease of shale density, consequent increase of shale porosity and lower associated water salinity in contrast to shales that had been buried under overpressured regime [6,7]. This is important because of the great expense of drilling and complexities in completing wells in a deep zones and prediction of sealing properties in deep formations. The "safe window" for drilling could be determined between the calculated pore and fracture pressures. For the well to be stable and under control, the mud pressure gradient must always be in between. There are several mechanisms of the abnormal formation pressure (AFP) forming which are discussed in literature [3,4,7,8]. This includes: buoyancy; compaction; diagenesis; hydrocarbons generation; osmosis; temperature and tectonic. And also Adam Bourgoyne adds fluid migration across faults or underground blowout [1].
Compaction of sediments with increasing depth of burial is believed to be the main source of abnormal pressure [4]. After the initial effect of compaction causing expulsion of interpore water from the shaly rocks, diagenetic changes become more important. During alteration, clay minerals are transformed from low density, highly disordered products of weathering/reworking into dense chlofite [2]. The difference in compaction can typically be observed and quantified using acoustic, neutron, density and resistivity logs.
