Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
1999-07-23
2001-01-30
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06182013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the management of hydrocarbon producing wells. More particularly, the invention relates to methods and apparatus for dynamically mapping the location of an oil-water interface and for predicting reservoir fluid movement and pressures under different production conditions.
2. State of the Art
In a petroleum reservoir, oil is produced through a well under pressure of gas, water, or compaction. Water may be naturally present in the reservoir displacing the oil to urge it out through the well bore. Often, water is injected into the reservoir from an injection bore located near the production bore. As oil is extracted from the well, the water moves through the porous medium of the formation closer to the well and the oil-water interface changes shape. If the location of the oil-water interface is not monitored during production, it is possible that the well will produce a mixture of oil and water. In some cases, it is possible for the well to produce more water than oil.
Well logs are a primary source of information used to map the distribution of fluids in hydrocarbon reservoirs. Because of the high electrical resistivity of hydrocarbons compared to formation water, open hole well logs of resistivity are typically used to infer water saturation, the percentage of pore volume occupied by water. As wells are typically cased with conductive steel pipe after drilling, it is not usually possible to take resistivity measurements through the casing. If a non-conductive casing is used, crosshole tomography techniques can be used to map the distribution of electrical resistivity in the reservoir volume. Measurement of fluid pressures is also used to estimate multiphase fluid flow properties (e.g. water and oil mobilities) and the location of the oil-water interface.
Previously incorporated U.S. Pat. No. 5,467,823 and U.S. Pat. No. 5,642,051 disclose methods and apparatus for monitoring a production reservoir with pressure and resistivity sensors which are permanently mounted in the production well between the casing and the borehole. The '823 patent does not specifically address the issue of monitoring the location of the oil-water interface and neither patent discloses any method for interpreting data acquired by the sensors in order to predict the location of the oil-water interface over time.
Previously incorporated U.S. Pat. No. 5,767,680 discloses a method for sensing and estimating the shape and location of oil-water interfaces in a formation traversed by a well. The method includes making time-lapse DC/AC measurements with an array of permanently deployed sensors in order to detect and estimate the change in geometry and proximity of the oil-water interface as a result of production, and therefore as a function of time. The estimation is carried out with a parametric inversion technique whereby the shape of the oil-water interface is assumed to take the form of a three-dimensional surface describable with only a few unknown parameters. A nonlinear optimization technique is used to search for the unknown parameters such that the differences between the measured data and the numerically simulated data are minimized in a least-squares fashion with concomitant hard bound physical constraints on the unknowns. The estimation procedure is robust in the presence of relatively high levels of noise and can therefore be used to anticipate deleterious water breakthroughs, as well as improve the efficiency with which the oil is produced from the reservoir.
The fundamental challenge posed in interpreting reservoir property measurements is to optimize each measurement of reservoir properties at time t by using all of the measurements acquired up until time t. This is difficult because the relationship between measurements and reservoir properties is generally complex and because measurement errors will affect subsequent interpretations of reservoir properties.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide methods for interpreting measurements made in a producing hydrocarbon reservoir to estimate the distribution of fluids and the multiphase flow properties of the reservoir.
It is also an object of the invention to provide methods for estimating the location of the oil-water interface in a producing hydrocarbon reservoir.
It is another object of the invention to provide methods for optimizing reservoir property measurements with prior information and previously acquired measurements.
It is still another object of the invention to provide apparatus for performing the methods of the invention.
In accord with these objects which will be discussed in detail below, the methods of the present invention include taking resistivity and pressure measurements in a producing hydrocarbon reservoir over time and interpreting the measurements to determine the distribution of fluids and the multiphase flow properties of the reservoir and the location of the oil-water interface. The measurement sensors may be located in the injecting well, in the production well, or in a dedicated monitoring well. The pressure measurements may be of fluid pressure in a well or pore pressure in the formation. The resistivity measurements may be DC potential measurements made with electrodes or AC electromagnetic measurements made with antennae. According to a preferred aspect of the invention, the sensors are arranged as distributed arrays and water injection is periodically interrupted while fall-off pressure and resistivity are monitored.
The pressure and resistivity measurements may be interpreted in several ways. Preferably, for each estimate made in the interpretation of a data set, a measure of the accuracy of the estimate is also made. Estimates and their “uncertainties” are then used to compute forecasts of reservoir performance and the uncertainties of the forecasts. The forecasts allow the oil field operator to optimize reservoir production by varying the injection/production rates to minimize or eliminate water production.
According to a first method, resistivity and pressure measurements are acquired simultaneously at an observation well during a fall-off test. Resistivity measurements are used to estimate the radius of the water flood front around the injector well based on known local characteristics. The flood front radius and fall-off pressure measurements are used to estimate the mobility ratio. According to a second method, resistivity and pressure measurements are acquired at a variety of times at an observation well and pressure measurements are taken during fall-off tests. Prior to taking any measurements, knowledge about the reservoir parameters is quantified in a prior probability density function (pdf). Applying Bayes' Theorem, the prior pdf is combined with measurement results to obtain a posterior pdf which quantifies the accuracy of the additional information. As new measurements are acquired, posterior pdfs, updated for expected temporal variations, become prior pdfs for the new measurements. According to a third method, uncertainty about the reservoir parameters is represented by Gaussian pdfs. The relationship between measurements and reservoir parameters is approximated by a linear function. Uncertainties are quantified by a posterior covariance matrix.
REFERENCES:
patent: 5467823 (1995-11-01), Babour et al.
patent: 5642051 (1997-06-01), Babour et al.
patent: 5767680 (1998-06-01), Torres-Verdin et al.
Abbaszadeh, M. and Kamal, M.Pressure-transient Testing of Water-Injection Wells.SPE Reservoir Engineering (Feb. 1989) pp. 115-124.
Adolph, B. et al.Saturation Monitoring With the RST Reservoir Saturation Tool.Oilfield Review, 6 (Jan. 1994) pp. 29-39.
Bard, Yonathan.Nonlinear Parameter Estimation.Academic Press, New York. (1974) Chapter V, Computation of the Estimates I: Unconstrained Problems. pp. 83-140.
Ellis, Darwin V.Well Logging for Earth Scientists.Elsevier, New York. (1987) Chapter 20, Saturation Estimation. pp. 471-489.
Jazwinski, Andrew H.Stochastic Processes and Filtering
Bryant Ian
Chen Min-Yi
Karakas Metin
Malinverno Alberto
Rossi David J.
Batzer William B.
Gordon David P.
McElheny Jr. Donald E.
Schlumberger Technology Corporation
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