Seismic pressure-prediction method solves problem common in deepwater Gulf of Mexico.

THE AUTHORS

R. Wilhelm retired from shell Oil Co will 20 years of experience and now a consulting geophysicist. He holds an MS on physics from the University of Texas and an MS in petroleum engineering from Tulane University.

L. B. Franceware is a geophysicist in the Deepwater E&P Division of Sheet Offshore He has been with Shell Oil Co since 1972 and holds a BS and MS in physics from the University of Texas.

Carlos P. Guzman is a geophysicist on the New Ventures Tean Deepwater E&P Division of Shell Offshore. He has worked with Shell Oil Co. since 1977. He holds a BS in physics from the University of New Orleans and an MS in physics from Purdue University.

A novel method of predicting pore from pressure gradients seismic velocities has overcome a problem common in the deepwater Gulf of Mexico.

The method, based on a model of geopressures that uses available formation temperature measurements, does not require the normal shale compaction trend. This is crucial in deep water, where geopressures often start shallow below mud line and make establishing the normal trend difficult.

Pore pressure prediction methods that use the shelf's depositional model are not accurate enough for the deep water. The shelf model consists of a significantly thick, hydrostatically pressured section, which is used to establish the normal pressure interval travel time (ITT)-shale compaction trend as a reference.

This article describes the method, discusses preparation of the velocity data, and shows results from one shelf and three deepwater wells in which the method is successful.

Seismic velocities

Seismic velocities are used to predict pore pressure in planning and designing wells. Estimation of the onset of geopressure from velocities is not new. Many workers currently use seismic velocities for qualitative pore pressure prediction.

The general approach is based on the observation that anomalously slow velocities are usually associated with higher pore [pressures.sup.(1)(2)]. Other workers have made extensive use of well log velocity data for geopressure [analysis.sup.(3)(4)(5)(6)(7)] and prediction of fracture pressure [gradients.sup.(5)(6)(8)(9)(10)].

In deep water, the deltaic model of a significantly thick hydrostatically pressured section followed by a rapid change into geopressures often does not hold. The start of geopressure often occurs shallow below mud line and is followed by a pressure profile that is more variable and less predictable than profiles typical of shallow water.

In order to increase predictability, one needs to exploit the information inherent in seismic velocity data before drilling. We use seismic velocity data as input to a constitutive equation to predict pore pressures.

A prerequisite for standard fluid pore pressure and fracture gradient prediction methods is the normal (hydrostatic) interval travel time-shale compaction trend that is used as a reference. The early onset of pressures in deep water results in a scarcity of ITT measurements that correspond to the normal pressure. Therefore, determination of the normal shale compaction trend has become more tenuous.

Pore pressure computation

We have implemented and studied two computational methods to transform geophysical velocity data to pore pressure, and pore pressure back to velocities: Eaton's method and Dutta's method.

Both methods make use of Terzaghi's effective stress approach, in which the effective stress gradient (VES in psi/ft) is computed from the seismic interval velocity (or its inverse, the ITT). The result is subtracted from the overburden stress gradient (OBG in psi/ft) to obtain the fluid pore pressure gradient (FPG in psi/ft):

FPG=OBG-VES. …