Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-11-13
2004-09-14
Hoff, Marc S. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C702S006000, C702S013000
Reexamination Certificate
active
06792354
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method to determine quantity, distribution, and speed of recovery of hydrocarbons in oil and gas subterranean reservoirs. More particularly, the present invention is a method for using petrophysical data from a plurality of wells, in a plurality of reservoir regions, containing a plurality of reservoir rock types, in the context of a three dimensional geological model, for identifying Dimensionless Capillary Pressure Functions and using these Dimensionless Capillary Pressure Functions for determining reservoir fluid volumes, fluid contacts, the extent of reservoir compartmentalization, and an improved estimate of reservoir permeability.
Aspects of the present invention draw from the fields of geology, geophysics, petrophysics, petroleum engineering, and applied mathematics. The present invention relates to methods for assisting engineers, geologists, and others to address the following key issues in the development of oil and gas reservoirs: the calculation of the distribution and volume of hydrocarbons in place, the degree to which reservoir fluids are contained in isolated flow compartments, and the speed with which fluids may be recovered. The first issue concerns the concepts of porosity and fluid saturations, which are, respectively, the fraction of the rock volume available for reservoir fluids and the fraction of the pore space containing a particular fluid. The second issue concerns the concept of reservoir compartmentalization, the degree to which reservoir fluids flow in isolated flow units. The third issue concerns the concept of permeability, a parameter that relates fluid flow rates to imposed pressure gradients, which can be due to injection of fluids or to natural conditions such as aquifers or gas caps. The present invention relates to a method for using Dimensionless Capillary Pressure Functions as derived from well logs to calculate saturations and fluid contacts, identify reservoir compartments, and improve on estimates of permeability in three-dimensional geological models.
2. Prior Art
In general, the present invention is a method for assisting engineers, geologists, and others associated with the development of oil and gas subterranean reservoirs to address questions concerning how much hydrocarbons are in a given location, how they are distributed within that location, and how fast can they be recovered from this given location. The present invention helps to address all three of these questions whereas the prior art has known disadvantages.
It is well known that reservoir fluids are distributed according to the interplay of gravitational and capillary forces. Capillary pressure curves, which describe capillary forces, are typically measured in laboratory experiments. In one such type of an experiment, a completely water-saturated rock is exposed to oil. Typically, in the case of water-wet rock, oil does not enter the pore space of the rock until a certain pressure, referred to as the displacement pressure or entry pressure, is exceeded. As the oil pressure is increased above the entry pressure, more and more oil enters the pore space and a corresponding amount of water leaves the pore space. As pressure continues to increase, it becomes increasingly difficult to remove water: there is proportionately less and less water leaving the rock. Eventually, at high oil pressures, a low saturation of water remains. This water saturation is referred to as the irreducible water saturation. The shape of the capillary pressure curve is an indicator of the distribution of the sizes of the pores within the porous media. Thus, rocks of various porosities and permeabilities exhibit widely varying capillary pressure curves. Thus, capillary pressure curves are used to characterize reservoir rocks.
Reservoirs contain various compositions of oil, water, and gas that are distributed within heterogeneous rocks, exhibiting a high degree of variability in porosity and permeability. For example, if the reservoir were deposited within a fluvial (relating to ancient rivers or channels) environment, there is typically a high degree of heterogeneity, both in an inter-channel and intra-channel areas. On the larger, inter-channel scale, the channels would exhibit high permeabilities; whereas, intervening flood plain deposits would be of lower permeabilities, and intervening shales would exhibit little or no permeability. On the intra-channel scale, one might encounter highly heterogeneous porosities and permeabilities in mud clast rocks at the base of a channel (where the ancient rates of sediment transport were highest). Higher up in a given channel, one might encounter plane and cross-bedded rocks exhibiting high permeabilities. These might grade up into finely sorted ripple laminated facies exhibiting uniform, but lower, permeabilities. Heterogeneous reservoirs, whether they are comprised of sandstones, carbonates, or other types of rocks are the norm.
In general, two rock samples from the same reservoir will have different capillary pressure curves when their permeabilities and/or porosities are different. Consequently, given the typical high degree of reservoir heterogeneity, a representative characterization of reservoirs using capillary pressure curves is likely to be an arduous task. Complex geological models containing in excess of a million cells can, in principle, require millions of measurements to describe the necessary capillary pressure curves.
More than fifty years ago, Leverett identified a way around this problem by proposing a dimensionless capillary pressure curve called the J Function,
J
(
S
w
)
=
P
c
k
/
φ
σ
cos
θ
See Leverett, M. C., “Capillary Behavior in Porous Solids”, Transactions of the AIME 142, 152-169 (1941). In this equation, S
w
denotes water saturation; P
c
, capillary pressure; k, permeability; &phgr;, porosity; &sgr;, interfacial tension; and &thgr;, contact angle. Upon analyzing experimental data, Leverett discovered that many rock samples, within a certain rock type or classification, exhibited one characteristic curve instead of multiple capillary pressure curves. Thus, the advantage of his approach is that many rock samples exhibiting various porosities and permeabilities are, within a particular rock type, classifiable by a single curve. Consequently, the problem of describing millions of capillary pressure curves for a geological model can be reduced to using a manageable number of Leverett J Functions. Typically, Leverett J Functions are correlated with parameters such as lithology, shale volume, and reservoir zone.
The present invention generalizes the equation of the Leverett J Function into a function that is, heretofore, referred to as the Dimensionless Capillary Pressure Function. It is defined as follows:
J
(
S
w
)
=
P
c
k
σ
cos
θ
f
(
φ
)
Like Leverett's J Function, this new function is dimensionless. In this equation, ƒ denotes a function of porosity, thus generalizing the dependence on porosity and reducing to Leverett's result when the function ƒ is the reciprocal of the square root. Leverett defined the function as &phgr;
−n
where n=½ as is depicted in the previous formula. It is contemplated that the variable n may be any other positive number as depicted.
There are known expensive and time consuming methods which attempt to determine aspects of subterranean formations for evaluation of oil and gas recovery with various application identifying Dimensionless Capillary Pressure Functions from laboratory measurements on core samples. The latter refers to small segments of reservoir rock that are recovered from wells. Oftentimes, due to the additional costs of coring operations, core samples are not recovered from many oil and gas wells. Typically, when they are recovered, they represent a sparse sampling of reservoir rocks within a well. Consequently, Dimensionless Capillary
Gutierrez Anthony
Hoff Marc S.
Ozinga Martin G.
Phillips McFall McCaffrey McVay & Murrah
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