Relationship Between Hydrocarbon Accumulation and Geopressure and Its Economic Significance
Authors: Timko D.J. , Fertl W.H. 
Published: December 2007

Log-calculated formation pressure data, reflecting the favorable or unfavorable nature of hydrocarbon environments, can be used to predict whether accumulations are commercial or noncommercial and also the type of hydrocarbon present. This knowledge is vital in making such economic decisions as whether to set pipe and continue drilling or to plug and abandon and turn the money to better use.

Introduction

Many published descriptions of drilling and production problems in overpressured formations - and probably many more that have not been widely published - demonstrate that geopressures are apparently not peculiar to any specific part of the world. In a published paper we have already discussed areas published paper we have already discussed areas of geopressure that had been reported earlier. Scattered throughout the literature are well data presenting maximum geopressures encountered in the drilling for and production of oil and gas. It is not in the scope of this paper to present a complete or even partial listing of these data. However, in connection with our present study it is of special interest to take a closer look at the magnitude of geopressures encountered in the Gulf Coast over the last few years since improved drilling techniques have enabled us to drill more efficiently in these highly pressured environments. Such environments and areas have been avoided previously because of associated drilling and previously because of associated drilling and completion problems and high costs. As early as 1938, Cannon and Craze presented the relationship of pore pressure to depth in the Gulf Coast area of Texas and Louisiana, reporting maximum pressure gradients approaching 0.765 psi/ft. A few years later, in 1943, in a study of abnormal saltwater pressures on the Texas and Louisiana coast, a maximum pressure gradient of 0.83 was quoted by Denton. Only 3 years later, in subsequent observations in the Gulf Coast area by Cannon and Sullins the estimate of maximum pressure gradient was increased to 0.865 psi/ft. Dickinson, in 1953, quoted values as high as 0.872 psi/ft, and concluded that these gradients appear to psi/ft, and concluded that these gradients appear to indicate that the upper limit of abnormal pressure gradients is being approached, and that it is unlikely that it will exceed about 0.90 pound per square inch per foot depth. per foot depth. Since then several oil companies have encountered higher pressure gradients than quoted by Dickinson; however, most of the literature indicates that the majority of the commercial hydrocarbon accumulations still appear to be associated with geopressures up to about 0.85 psi/ft. For example, Fowler states that the "only systematic difference observed between commercial, and barren or non-commercial, abnormally pressured pay sands in Chocolate Bayou [Brazoria County, pressured pay sands in Chocolate Bayou [Brazoria County, Tex.] is a difference in pressure gradient. Sands with commercial hydrocarbon accumulations have pressure gradients less thin about 0.85 psi/ft; sands with pressure gradients greater than about 0.85 psi/ft, do pressure gradients greater than about 0.85 psi/ft, do not contain commercial hydrocarbons."A quite similar high pressure gradient has been reported in a reservoir on the north flank of the Egan field, Acadia Parish, La. With an initial pressure gradient of 0.84 psi/ft, the subject reservoir produced 6 billion cu ft of gas, 170,000 bbl of condensate and 1 million bbl of salt water over a period of 6 1/2 years. More recently, Classen published some formation pressure-production relationships for the area around pressure-production relationships for the area around the Lake Mongoulois dome in central St. Martin Parish, La. Parish, La.

JPT

P. 923