Uses of Pressure and Temperature Data in Exploration and Now Developments in Overpressure Detection
Authors: Pikington P.E. 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Pressure and temperature data from wireline logs andseismic-velocity analysis can be used in deep overpressures to aid in prospectevaluation. Properly interpreted seismic-velocity analysis in deep water canyield maturation and migration information. Use of a modified approach to the dexponent and use of measurement-while-drilling (MWD) data have provedsuccessful in deepwater pressure detection.

Introduction

Abnormal pressure prediction, detection, and confirmation have been in usefor a number of years. Ho and Johnson's and Wallace, papers in 1965 originateda whole new technology with great economic value that is difficult to assess interms of dollars. Pore-pressure work is similar to corrosion engineering. Noone pays attention as long as everything is working, but everyone notices ifsomething goes wrong and a disaster occurs. Pore-pressure and corrosionengineering are designed to eliminate the cost of fighting a problem.

Pressure-detection work started with the d exponent and confirmationtechniques with wireline resistivity, conductivity, and acoustic togmeasurements. Cuttings density, gas, and flowline temperature followed asdetection parameters. Prediction from seismic-velocity analysis also wasdeveloped. The basic methods apply to many enshore and offshore basinsthroughout the world; however, the pressure-estimating techniques vary frombasin to basin and sometimes within a basin.

This paper presents an overview of the traditional methods used, as well asnew concepts that apply to the use of pressure and temperature data toexploration. These data can be used in preacquisition evaluation as anothermeans of rating prospects. The methods described can be used to re-evaluatecurrently owned acreage before committing to drill deep, costly exploratoryholes.

The techniques discussed can be applied to clastic basins, where thickoverpressured shale sections occur, in addition to the Gulf of Mexico. In theGulf of Mexico, the top of overpressure varies from 18,000 ft [5486 m] belowthe mudline to less than 1,000 ft [less than 305 m] below the mudline. Depth tothe top of overpressure generally decreases with distance from the depocenterin any overpressured basin, and the magnitude of the pressure gradient at anygiven depth increases. This is a direct result of the decrease in the volume ofpermeable sediment deposited farther from the shoreline.

Traditional Methods

Pore-pressure detection with drilling parameters works well in clasticbasins. Fig. 1 is a schematic showing the response of various parameters thatcan be measured during drilling. Rate of penetration (ROP) shows an increasewhen a transition zone in shale is encountered during drilling with stabledrilling parameters (bit weight, rotary speed, mud weight, and hydraulics). Theincrease in ROP with rock bits is caused by the increase in fluid content ofthe shale below the pressure seal (or caprock) plus the decrease inoverbalance. The d exponents is a normalizing technique that minimizes theeffect of changing drilling variables, and the modified d exponent, d, can bequantified empirically for estimating pressures. There is no dc-vs.-pressurecorrelation that works everywhere, but the matrix-stress technique forestimating overpressures can be used in many areas when used properly. Weightper inch of bit diameter, W/D, should also be monitored bemuse large changes inW/D on a bit run can exceed the ability of the d c exponent compensate and atrend shift may be required. Other "on-bottom" indicators that are used includetorque and fill, which may increase when the well becomes underbalanced andshale sloughing occurs. The final on-bottom indicator used is a flow check.

Other parameters can be monitored as the mud is circulated to the surfaceand then properly lagged to provide data that are "behind" tie bit. Lithologymust be monitored and is a gross indicator of a possible pressure transitionfrom nominal to above-normal pore pressure. No transition zone is likely tooccur when porous sediments dominate the lithology column.

JPT