RIASSUNTO
Abstract
Tight oil and gas reservoirs often require stimulation in order to make economic exploitation possible. Tailored pulse loading, usually by possible. Tailored pulse loading, usually by downhole use of propellants or explosives, has been studied as a means of improving production in these reservoirs. This paper presents an overview of the first year's effort on a three-year combined theoretical-experimental research program to develop a systematic understanding of the tailored pulse loading/fracturing phenomena. The project pulse loading/fracturing phenomena. The project is designed to address the sensitivity of multiple fracture initiation and propagation to rock and fluid properties and pressurization history. The results will ultimately contribute to the design procedures for field applications. procedures for field applications
Introduction
Many hydrocarbon-bearing reservoirs are insufficiently permeable to allow economic production by conventional well completion techniques. production by conventional well completion techniques. Examples are the tight sands of the western U.S. and the Devonian shales of the eastern U.S. Aside from formation characteristics, the deliverability of hydrofractured wells tend to be proportional to fracture conductivity and fracture area within the pay zone. A method to optimize these parameters pay zone. A method to optimize these parameters which has been seriously studied for well workovers, overcoming wellbore damage, and stimulation of naturally fractured formations is tailored pulse loading. Using a sufficiently fast pulse loading. Using a sufficiently fast pressurization rate, typically with a propellant, pressurization rate, typically with a propellant, multiple radial fractures from the wellbore can be achieved. One can easily envision that the possibility of linking a natural fracture set to the possibility of linking a natural fracture set to the wellbore is enhanced using tailored pulse loading as compared to a conventional biwing hydraulic fracture. With the exception of some limited empirical models, little is known of the physical phenomena and parameters governing the multiple phenomena and parameters governing the multiple fracturing process as caused by tailored pulse loading. In particular, the process is likely to be sensitive to downhole stress conditions, formation elastic and hydraulic parameters, fluid parameters, and pressurization history. This parameters, and pressurization history. This paper describes a three-year program to address in paper describes a three-year program to address in detail the sensitivity of the multiple fracturing phenomenon to these parameters and the first phenomenon to these parameters and the first year's results of that program.
Multiple fracture from a pressurized wellbore can be considered to occur in three sequential stages. The first stage is the wellbore pressurization prior to rupture. During this stage, pressurization prior to rupture. During this stage, pressure diffusion from the wellbore is occurring pressure diffusion from the wellbore is occurring simultaneously with wellbore deformation. Flaws, which may be either microfractures or preexisting macro-fractures, are undergoing pressurization at the wellbore and energy is being stored in the wellbore either by liquid or gas compression.
The second stage begins at the time of flaw rupture with subsequent extension from the wellbore. In this stage, the fractures are sufficiently short to propagate independently -- that is, each fracture is blind to the existence of the other fractures. The fractures do not begin to effectively ""feel"" the other fractures until they have extended one or two well radii from the well face. During this second stage, the large amount of energy stored in the compressed wellbore fluid at the time of rupture is released. Since this is a wellbore storage phenomena it is very sensitive to the volume of the packed-off section of the wellbore and to the fluid compressibility.
The last stage of the process is the extension of multiple fractures from the wellbore, during which fracture-fracture interaction is occurring. During this stage, the fracturing process is very sensitive to the fluid pressure process is very sensitive to the fluid pressure distribution within the fractures. Analytic solutions for stress intensity which assume a uniform pressure loading within the fracture indicate that, if any of the fractures become longer than the others, those fractures will continue to propagate and the shorter fractures will halt due to clamping by the longer fractures. Thus, only a single biwing fracture will result.
P. 87