Tuesday, April 3, 1:00pm

Mr. Adrian White

PhD Candidate, Dept. Geological Sciences, University of Durham, United Kingdom.
 

Driven by overpressure: The shameful confessions of a crack addict
 
 

Abstract

The fracture gradient is an important parameter in drilling because in most cases it defines the upper limit to pore pressure (Pp). Knowledge of the fracture gradient can be used to improve drilling safety and reduce the risk of "blowouts". Empirical fracture gradient determination methods are available but pumping pressure test data also provide a fairly reliable means of estimating its magnitude. Plotting leak-off pressure (LOP) data on stress-depth plots produces an estimate of minimum in-situ stress (S3) with depth. This has been done for seven contrasting basins: Mid-Norway, the Central North Sea, the Gulf of Mexico, Brunei, Onshore Nigeria, Offshore West Africa and the Barents Sea. In each instance it was assumed that the lower bound to LOP data is a proxy for the S3. Lower bound trends were determined using:

(a) The entire LOP data set;
(b) A data subset where LOP and Pp were recorded within 100m of each other in the same well.

Results show that having more data points for each basin helps to better define the lower bound trend. Also noted was the tendency for lower bound trends to "curve" towards the lithostat with depth. From the basins analysed, there does not appear to be a relationship between tectonic regime and the shape of the lower bound trend. The Mid-Norway data set was used to test the reliability of LOPs for use in determinations of S3 magnitude. Results show that LOPs can produce a good estimate. The commonly assumed better proxies, instantaneous shut-in pressure (ISIP) and fracture closure pressure (FCP), differed only slightly from the LOP in Mid-Norway. Since the majority of the S3 determination comes from the pressure exerted by the static mud column, the small difference between LOP, ISIP and FCP was shown to make little difference to results.

Published compilations of LOP and Pp data suggest a linkage in which it appears that there is a pore pressure in-situ stress coupling. This is revealed in overpressured regions where the magnitudes of the horizontal stresses are greater than expected. Explanations for coupling include a change in tectonic regime with depth, poro-elasticity and frictional limits to stress. For this study, LOP data where paired with Pp data recorded in the same well and within 100m of the leak-off test thus testing the strength of the coupling relationship. These subsets of the data were to provide insights into the degree of coupling not shown on stress-depth plots in two ways:

(a) By plotting overpressure (Pp - hydrostatic P) against the magnitude of Sh above Sh(T)
Where: Sh(T) = 70% SV (when Pp is hydrostatic);
(b) By plotting Sh and Pp as gradients.

Results show coupling can be inferred in four of the seven basins. Poro-elasticity or frictional limits to stress are the most likely explanations because regional S3 gradients do not appear to relate to tectonic regime. Coupling is not displayed in either the Gulf of Mexico or Mid-Norway data sets. Reasons for the unexpected results from Mid-Norway will be the subject of further study. The lack of overpressures in the Barents Sea precludes any coupling relationship. It therefore appears that Pp-Sh coupling supports a higher fracture gradient with overpressure. This has implications for well planning in overpressured regions.

Biography of the Speaker

Graduated from the University of Durham with a Bachelor of Sciences Honours degree in Geology in July 1998. Went straight into a Ph.D. entitled "Fractures as a control on pore pressures in sedimentary basins" supervised by Dick Swarbrick at the University of Durham in October 1998. His Ph.D. is a small part of the GEOPOP (GEOsciences Project into OverPressure) research project. This is a multi-disciplinary industry funded research group investigating the development and consequences of overpressure and involves Durham, Newcastle and Heriot-Watt Universities. Research interests include pore pressure in-situ stress coupling (the reason for being here sunning it up) and most aspects of structural geology. His insatiable appetite for geology in general has lead to his ambition to go on every undergraduate fieldtrip Durham University has to offer. This talk is a brief summary of results to date.