Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2002-09-06
2004-01-06
McElheny, Jr., Donald E. (Department: 2802)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06675097
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to processing gravity and magnetic data using vector and tensor data along with seismic data and more particularly to the inversion of gravity and magnetic data and combining with seismic data for hydrocarbon exploration and development, and to detect abnormally pressured subterranean formations in general, and with specific application to areas underneath and around anomalies such as salt, igneous or magmatic formations.
2. Related Prior Art
In exploration for hydrocarbons in subsurface environments containing anomalous density variations has always presented problems for traditional seismic imaging techniques by concealing geologic structures beneath zones of anomalous density. Many methods for delineating the extent of the highly anomalous density zones exist.
U.S. Pat. No. 4,987,561, titled “Seismic Imaging of Steeply Dipping Geologic Interfaces, issued to David W. Bell, provides an excellent method for determining the side boundary of a highly anomalous density zone. This patent locates and identifies steeply dipping subsurfaces from seismic reflection data by first identifying select data which have characteristics indicating that seismic pulses have been reflected from both a substantially horizontal and a steeply dipping interface. These data are analyzed and processed to locate the steeply dipping interface. The processed data are displayed to illustrate the location and dip of the interface. This patent, while helping locate the boundaries, provides nothing to identify the subsurface formations on both sides of the boundary.
There have also been methods for identifying subsurface formations beneath anomalous zones using only seismic data to create a model and processing the data to identify formations in light of the model. By further processing reflection seismic data, the original model is modified or adjusted to more closely approximate reality.
An example of further processing seismic data to improve a model is U.S. Pat. No. 4,964,103, titled “Three Dimensional Before Stack Depth Migration of Two Dimensional or Three Dimensional Data,” issued to James H. Johnson. This patent provides a method of creating a three-dimensional model from two dimensional seismic data. This is done by providing a method of ray tracing to move before stack trace segments to their approximate three-dimensional position. The trace segments are scaled to depth, binned, stacked and compared to the seismic model. The model can then be changed to match the depth trace segments that will be stacked better, moved closer to their correct three-dimensional position and will compare better to the model. This patent uses a rather extensive seismic process to modify a seismic model that may be inaccurate.
One source of geologic exploration data that has not been used extensively in the past is potential fields data, such as gravity and magnetic data, both vector and tensor data and using potential fields data in combination with seismic data to provide a more accurate depth model or to derive a velocity model.
Gravity gradiometry has been in existence for many years although the more sophisticated versions have been held as military secret until recently. The measurement of gravity has become more acceptable in the late eighteen hundreds when measuring instruments with greater sensitivity were developed. Prior to this time, while gravity could be measured, variations in gravity caused by the effect of a large nearby object at one location, the gravity gradient, could not be reliably measured.
It has been known since the time of Sir Isaac Newton that bodies having mass exert a force on each other. The measurement of this force can identify large objects having a change in density even though the object is buried beneath the earth's surface or in other ways out of sight.
Exploration for hydrocarbons in subsurface environments containing anomalous density variations such as salt formations, shale diapers and high pressure zones create havoc on seismic imaging techniques by concealing geologic structures beneath zones of anomalous density. By utilizing gravity, magnetic and tensor gravity field measurements along with a robust inversion process, these anomalous density zones can be modeled. The spatial resolution obtained from this process is normally much lower resolution than that obtained from reflection seismic data. However, models obtained from gravity and magnetic data can provide a more accurate starting model for the seismic processing. Using the potential fields data models as a starting point for two dimensional and three dimensional seismic depth imaging greatly enhances the probability of mapping these concealed geologic structures beneath the zones of anomalous density.
Zones of anomalous density may also be associated with zones of anomalous fluid pressure. Typically, while drilling an oil or gas well, the density of the drilling mud must be controlled so that its hydrostatic pressure is not less than the pore fluid pressure in any formation along the uncased borehole. Otherwise, formation fluid may flow into the wellbore, and cause a “kick.” Kicks can lead to blowouts if the flow is not stopped before the formation fluid reaches the top of the well. If the fluid contains hydrocarbons, there is a serious risk of an explosion triggered by a spark. For this reason, wellbores are drilled with a slight excess of the borehole fluid pressure over the formation fluid pressure.
A large excess of the borehole fluid pressure over the formation fluid pressure, on the other hand, is also undesirable. Fractures in the borehole wall may result in loss of circulation of the drilling fluid, resulting in stuck drill strings. Even if drilling can be continued, it is slowed down, resulting in greater costs. Serious formation damage may also occur.
Pressure prediction is done by estimating certain key parameters that include the overburden stress or confining stress, which is defined as the total lithostatic load on a rock volume at a given depth, and the effective stress, which is defined as the net load on the grain framework of the rock at a given depth. These two relations are then used in the Terzaghi effective stress law to estimate the fluid or pore pressure. Terzaghi's law states that:
Pc=Pe+Pp
where:
(Pc)=the confining stress
(Pe)=the stresses born by the grains, and
(Pp)=the stress born by the fluid.
Some workers treat a special case of Terzaghi's law where the confining stress is assumed to be the mean stress as opposed to the vertical confining stress. It should be acknowledged that this difference exists, but that it does not effect the embodiments of the present invention as they will pertain to estimating the total overburden load, which can then be converted to either vertical confining stress or mean stress based on the stress state assumptions that are made. The current prior art used for estimating confining stress is to use a density log from a nearby calibration well and integrate the density data to obtain the overburden load. This calibration was then applied from the mudline down to depths usually beyond the depth of sampling to predict the overburden away from the calibration well.
It has long been recognized that velocities of seismic waves through sedimentary formations are a function of “effective stress,” defined as the difference between the stress caused by the overburden and the pore fluid pressure. A number of methods have been used to measure the seismic velocities through underground formations and make an estimate of the formation fluid pressure from the measured velocities. Plumley (1980) and U.S. Pat. No. 5,200,929 issued to Bowers, (the '929 patent) describe a method for estimating the pore fluid pressure at a specified location. The method also accounts for possible hysteresis effects due to unloading of the rock formation. The method utilized a pair of sonic velocity-effective stress relations. One relationship is for formations i
Jorgensen Gregory Joseph
Kisabeth Jerry Lee
Routh Partha Sarathi
Conoco Inc.
Madan Mossman & Sriram P.C.
McElheny Jr. Donald E.
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