Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2000-04-19
2002-08-13
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06434487
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention is a method for estimating fluid pressures in a series of subterranean formations from formation interval velocity data. The invention measures directly, from the velocity data, the variations in the effective vertical stress, which are due to variations in the fluid pressures in the subterranean formations. The invention uses these variations to estimate the effective vertical stresses that, together with an estimate of the corresponding overburden stresses, allow estimation of the corresponding fluid pressures in the formations.
2. Related Art
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 fluid pressure present in the pores in any formation along the uncased borehole (where the borehole is open to the formations). Otherwise, formation fluid may flow into the borehole. This can lead to a blowout if the flow is not stopped before the formation fluid reaches the top of the well. If the fluid contains hydrocarbons, this can result in fire or explosion.
Blowouts result where the mud weight is too low to balance fluid pressures within the subterranean formations. Excessive overbalance, where the hydrostatic pressure of the drilling mud greatly exceeds the fluid pressures in the subterranean formations, can induce undesirable fractures in the borehole wall that causes loss of drilling fluid. The drill pipe may also get stuck along contact zones with the borehole wall if the hydrostatic pressure of the mud is too much in excess of the fluid pressure in the subterranean formations. Also overbalanced mud typically reduces the penetration rate of the drill bit resulting in increases in drilling time which result in increased drilling costs. Therefore, to optimize drilling performance and minimize drilling problems, the mud weight must be adjusted according to the variation of the fluid pressures in the formations along the borehole. Prediction of these variations in the fluid pressure in the formations along the borehole is essential to safe and economical drilling.
The prior art contains numerous discussions of the problem and of the geologic factors involved that create anomalous fluid pressures within the subterranean formations. Anomalous fluid pressures can be attributed to several causes.
The basic physics of determining pore pressure is described by Terzaghi's principle. Terzaghi's principle states that the total downward force on an element of volume of rock is supported by two upward forces, the effective vertical stress (the part supported by the rock matrix) and by the fluid pressure of the fluid in the pore space of the rock in said element of volume. Terzaghi's principal is expressed in equation form as:
S=P+EVS
where S=the downward force due to the weight of the overburden rock column,
P=the formation fluid pressure in the pore spaces of the rock, and
EVS=the effective vertical stress exerted upward by the rock matrix itself
Anomalous formation pressure can be caused by “undercompaction”, thermal expansion of the formation fluid trapped in the pore spaces of the formation (aquathennal pressuring), clay diagensis (expulsion/expansion of integranular water due to temperature changes) and various other causes. Undercompaction occurs when low permeability inhibits fluid in the pores of the formation from escaping as rapidly as the pore space would like to compact due to the force exerted by the weight of the column of rock above the formation.
The prior art contains numerous methods of computing the effective vertical stresses in the formations penetrated by the borehole, and subsequently computing the fluid pressure in those formations by subtracting the effective vertical stress, EVS, from the overburden stress, S. The prior art also contains numerous methods of computing pore pressure directly without having to compute effective vertical stress.
Most of the prior art techniques rely on either empirically derived baselines or empirically derived virgin curves in the form of interval velocity vs. depth. At least one prior art method expresses effective vertical stress in terms of temperature, age, rate of deposition, and other geologic parameters.
The technique described herein differs from previous methods and is useful because it can be employed, without detailed knowledge of the geological parameters at depth, to produce relative estimates of effective vertical stresses, and thus pore pressures, on an areal extent to the accuracy permitted by the input velocity functions. It is based upon the established fact that as the pore pressure changes in a given formation, the effective vertical stress changes, and thus the interval velocity changes in the formation. The pore pressure changes are thus reflected in the second derivatives of the velocity function changing as the second derivatives of the effective stress function changes at a given location.
The interval velocity information of the subterranean formations is usually obtained, at a specific location, from the sonic log obtained by passing a tool down the borehole and recording the interval transit times of sound passing through a given formation between a source and receiver within the tool. The interval velocities can then be directly calculated from the interval transit times. Also, the interval velocities of the subterranean formations can be obtained, both at the location to be drilled and over an areal map extent on a spatially varying basis by calculations on seismically recorded data.
This invention is unique in that it requires no baselines, virgin curves, nor detailed knowledge of geologic parameters at depth to compute, from the interval velocity vs. depth (or interval velocity vs. time) function, the effective vertical stress function. The only parameters needed are the water depth (for marine cases) and the initial conditions for at least the first depth interval of the effective vertical stress vs. depth to be computed plus the second derivatives of the interval velocity vs. depth friction. The first initial condition is that the effective vertical stress is zero at the mudline (for marine cases) or is zero at the surface for land cases. This is known to be true by definition. The second initial condition is to assume a rate of increase of effective vertical stress (a first derivative value) across the first depth (or time interval). For the depth case this first derivative is typically, but not limited to, the range of 0.465 psi/ft to 0.535 psi/ft. If the first interval is hydrostatically pressured (is in hydraulic communication with the surface) then theoretically this first derivative should be very close to 0.535 psi/ft (or the equivalent in time for the time case).
Generally sonic logs do not have sufficient information on the shallow data to compute the initial conditions directly from the data, the logs usually being started, especially in deep water, several thousand feet below the mud line. However, the newer generation of seismic 3D information usually provides rather good interval velocity information on the shallow zone below the mudline, and, when this is so, it is feasible to compute the initial conditions directly from the velocity data.
Since the second derivatives of the interval velocity function should be the same as the second derivatives of the desired effective vertical stress function, mathematics implies that an alternate solution using the first derivatives should be equally valid. This solution, in practicality, requires an accurate computation of the difference between the initial first derivative of the interval velocity function and the assumed first derivative of the effective vertical stress function as well as a knowledge, for marine cases, of the exact sediment velocity of the rock at the mudline.
The complete effect
Keeling Law Firm
McElheny Jr. Donald E.
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