Stimulated permeable fractures

COUPLED THM MODELING OF THE STIMULATED PERMEABLE FRACTURES IN THE NEAR WELL AT THE SOULTZ-SOUS-FORETS SITE (FRANCE)... 

Evans, K. F. (2000). The effect of the 1993 stimulations of well GPKl at Soultz on the surrounding rock mass Evidence for the existence of a connected network of permeable fractures., Proc. World Geothermal Congress 2000, Vol. 6, 3695-3670. 

During well completion it is sometimes desirable or necessary to treat or stimulate the producing zone to improve permeability of the rock and to increase the flow of oil or gas into the casing. This may be accomplished by use of acid or by injection of fluid and sand under high pressure to fracture the rock. Such a trcatniciit usually improves the ability of the rock to allow fluid to flow through it into the well bore. At this point the drilling and completion phases have ended. 

Hydraulic fracturing is a technique to stimulate the productivity of a well. A hydraulic fracture is a superimposed structure that remains undisturbed outside the fracture. Thus the effective permeability of a reservoir remains unchanged by this process. The increased productivity results from increased wellbore radius, because in the course of hydraulic fracturing, a large contact surface between the well and the reservoir is created. 

Most of these reservoirs have natural permeabilities below 10 mD, and the stimulation of their production is achieved through acid fracturing operations. Viscosified hydrochloric acid is pumped into wells at pressures larger than formation parting pressure. Irregular etching of the fracture walls by the acid is expected to create highly... 

Gidley, J.L., et al. "Stimulation of Low-Permeability Gas Formations by Massive Hydraulic Fracturing - A Study of Well Performance," J. Pet. Technol.(April 1979), 525-531. 

Agarwal, R.G., Carter, R.D., and Pollock, C.B. "Evaluation and Performance Prediction of Low-Permeability Gas Wells Stimulated by Massive Hydraulic Fracturing," J. Pet. Technol. (March 1979) 362-372. 

Hydraulic fracturing is a method of stimulating production of oil or gas from rock formations. A fluid is pumped under conditions of high pressure and high rate Into the formation to fracture it. The fluid also carries sand or a similar proppant material into the fractures. When the pumping is stopped and the hydraulic pressure is released at the wellhead, the fracture partially closes on the sand leaving a highly permeable channel for the oil or gas to flow back to the well. 

The production of tight gas is technically very demanding. The major differences from conventional production arise because of the poor permeability of tight reservoirs, where the natural gas cannot flow as quickly to the well or in sufficient volumes to be economic, and where production rates are usually quite low. The principal prerequisite for economically producing tight gas is, therefore, to improve reservoir permeability, e.g., by artificial stimulation techniques, such as hydraulic fracturing (i.e., the generation of artificial fracture systems). 

A foam, aqueous or non-aqueous, that is injected into a petroleum reservoir to improve the productivity of oil- or gas-producing wells. Some mechanisms of action for foam stimulation fluids include fracturing, acidizing to increase permeability, and diversion of flow. 

The key to a successful acid fradure stimulation is to control the fluid leak-off (23). Most fluid loss additives control leak-off by building up a low-permeability filter cake against the fracture face, thereby deating a wall-building mechanism. The three mechanisms that govern fluid loss as described by Howard and Fast (24) are... 

Foamed Matrix Acidizing. Matrix addizing is a stimulation treatment used to remove damage near the wellbore without deating a fracture. The process involves the injection of a reactive fluid into the porous medium at a pressure below the fracturing pressure. The fluid dissolves some of the porous medium and consequently increases its permeability. 

Foam stimulation fluids provide viable alternatives to stimulate low-pressure, fluid-sensitive gas wells where permeabilities are less than 5 md. Foams have been successfully used on shales, tight sandstones, and carbonates. Some interesting applications of foam fracturing have been on very sensitive formations and coal-bed methane fracturing. 

The experiments conducted so far at the site in range of depths between 3000 and 3800 m show that hydraulic stimulation techniques can considerably increase the permeability in a rock volume that can extend up to several hundred meters around the well, thereby serving to establish connections between the wells. The major impact of the hydraulic stimulation could be explained by hydromechanical mechanisms (ruptures in fracture planes). However in the near well at the scale of some meters, a decrease of the impedancy is outlined via hydraulic vibration tests and is presently not well understood. As at this scale, the hydromechanical effect cannot be dissociated from the thermal effect due to the injection of a cold water in a hot media, the thermo-hydro-mechanical responses of the stimulated fractures must be investigated. 

Cleats are natural opening-mode fractures in coal beds. They account for most of the permeability and much of the porosity of coal bed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations. Because permeability and stimulation success are commonly limiting factors in gas well performance, knowledge of cleat characteristics and origins is essential for successful exploration and production. 

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