Wells – Processes – With indicating – testing – measuring or locating
Reexamination Certificate
2002-02-14
2003-12-30
Neuder, William (Department: 3672)
Wells
Processes
With indicating, testing, measuring or locating
C166S250020, C166S307000
Reexamination Certificate
active
06668922
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to hydrocarbon well stimulation, and is more particularly directed to a method for designing matrix treatment, or generally any treatment with a fluid that will react with the reservoir minerals or with chemicals resulting for instance from a previous treatment. The invention is particularly useful for designing acid treatment such as for instance mud acid treatments in sandstone reservoirs.
2. Discussion of the Prior Art
Matrix acidizing is among the oldest well stimulation techniques. It is applied to sandstone formations to remove near-wellbore damage, which may have been caused by drilling, completion, production, or workover operations. Matrix acidizing is accomplished by injecting a mixture of aids (typically hydrofluoric and hydrochloric acids) to dissolve materials that impair well production, as a rule designated as near-wellbore damages.
Matrix treatments ma sandstone reservoirs have evolved considerably since the first mud acid treatment in the 1930s. Treatment fluid recipes have become increasingly complex. Several additives are now routinely used and organic acids are frequently used in high temperature formations to avoid precipitation reactions. Chelating agents are often added to avoid precipitation in formations with high carbonate content.
Substantial production improvements can be achieved by this type of well stimulation technique if treatments are engineered properly. However, matrix treatments are also often a main contributor to reservoir damages. Indeed, the side reactions that occur in almost all mud acid treatments, lead to the formation of precipitates. Precipitates plug pore spaces and reduce permeability and can therefore adversely affect acid treatments if precipitates deposit near the wellbore. Far from the well precipitates are considered to have negligible effect. Moreover, recent studies have made the industry wary of damage due to secondary and tertiary reactions. Accurate prediction of the effectiveness of a matrix treatment involves calculation of the rates of the dissolution and re-precipitation reactions of minerals because the rates dictate where precipitates will be deposited in the reservoir.
Moreover, sandstone mineralogy is quite complex and acid/mineral compatibility as well as acid/crude oil compatibility is often an issue. At present, there is a lack of tools that can predict accurately the reactivity of acids with clays, and consequently, there are treatments currently in practice that use empirical rules—or at the opposite extreme, rely on extensive costly and time-consuming laboratory testing.
Beyond the treatment fluid selection, the pumping schedule is also a crucial parameter. In The
Stimulation Treatment Pressure Record
-
An Overlooked Formation Evaluation Tool,
by H. O. McLeod and A. W. Coulter, JPT, 1969, p. 952-960, a technique is described wherein each injection stage or shut-in during the treatment is considered as a short individual well test. The transient reservoir pressure response to the injection of fluids is analyzed and interpreted to determine the conditions of the wellbore skin and formation transmissibility.
In
New Method Proves Value of Stimulation Planning,
Oil & Gas Journal, V 77, NO 47, PP 154-160, Nov. 19, 1979, G. Paccaloni proposes a method based on the instantaneous pressure and injection rate values to compute the skin factor at any given time during the treatment. Comparison is made with standard curves calculated for fixed values of skin effect to evaluate skin effect evolution during treatment. Standard curves are generated using Darcy's equations for steady state, single phase and radial horizontal flow in reservoirs.
A technique presented by Prouvost and Economides enables continuous calculation of the skin effect factor during the course of the treatment and accounts for transient response, see
Real
-
time Evaluation of Matrix Acidizing,
Pet. Sci, and Eng., 1987, p.145-154, and
Applications of Real
-
time Matrix Acidizing Evaluation Method,
SPE 17156, SPE Production Engineering, 1987, 4, No. 6, 401-407. This technique is based on a continuous comparison of the measured and presumed good reservoir description including the type of model and well and reservoir variables of the subject well.
It is also known from U.S. Pat. No. 5,431,227 to provide a method for matrix stimulation field monitoring, optimization and post-job evaluation of matrix treatments based on calculated and measured bottom hole pressure used in a step rate test to estimate the damage skin.
A number of sandstone acidizing models have been presented in the literature aiming at computing changes in porosity resulting from the dissolution and precipitation of minerals.
In the lumped mineral models, the complex sandstone mineralogy is lumped into characteristic minerals and an average reaction rate for these minerals is determined from core tests. In two mineral models the sandstone minerals are lumped into fast- and slow-reacting groups on the basis of their reactivity with HF. Two mineral models do not account for precipitation reactions. A three mineral lumped model has also been proposed in S. L. Bryant, SPE 22855, An Improved Model of Mud Acid/Sandstone Acidizing, in SPE Annual Technical Conference and Exhibition, 1991, Dallas. The third mineral accounts for the precipitation of amorphous silica. Disadvantages of lumped mineral models are that they do not allow for equilibrium reactions to be modeled and need to be carefully calibrated to the treatment condition and formation of interest. Therefore, these models are not applicable to fluids systems containing weak acids (e.g. most organic acids) and chelating agents and are not reliable outside the calibrated region.
The equilibrium approximation is another approximation that is frequently used for the design of matrix treatments. This model has been presented in Walsh, M. P., L. W. Lake, and R. S. Schechter, SPE 10625, A Description of Chemical Precipitation Mechanisms and Their Role in Formation Damage During Stimulation by Hydrofluoric Acid, in SPE International Symposium on Oilfield and Geothermal Chemistry, 1982, Dallas. In the equilibrium approximation it is assumed that the reactions are much faster than the contact time of the minerals with the acids. The equilibrium constants for the reactions are usually better known than the rate constants, so large reaction sets can be included and complex sandstone mineralogy can be accounted for without speculating on the reactions and rate laws as is necessary in the lumped mineral approach. Unfortunately, the assumption that the reactions are much faster than the contact time is not valid for the injection rates used in most acid treatments and thus the equilibrium approach is useful only as an indicator for precipitation. The question that must be answered for a successful design is not if but where precipitation will occur. An equilibrium model alone with no time dependence cannot answer this question.
To address this discrepancy in the equilibrium models, partial local equilibrium models have been proposed and first described in Sevougian, S. D., L. W. Lake, and R. S. Schechter, KGEOFLOW: A New Reactive Transport Simulator for Sandstone Matrix Acidizing, SPE Production & Facilities, 1995: p. 13-19 and in Li, Y., J. D. Fambrough, and C. T. Montgomery, SPE 39420, Mathematical Modeling of Secondary Precipitation from Sandstone Acidizing, SPE International Symposium on Formation Damage Control, 1998, Lafayette. The partial equilibrium approach combines the kinetic and equilibrium approaches. Slow reactions are modeled with a kinetic model, and an equilibrium model is used for fast reactions. This computation scheme enables comprehensive and flexible modeling of sandstone acidizing, but traditionally suffered from several disadvantages. First, accurate computation of the activity coefficients for high acidic and high ionic strength solutions is difficult. Second, due to inefficient numerical algorithms numerical converge
Robert Joel
Ziauddin Murtaza
Jeffery Brigitte
Menes Catherine
Ryberg John J.
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