Simulation of Produced Water Re-Injection Under Fracturing Conditions

Abstract

This paper was selected for presentation by an SPE Program Committee following rewew of mforma!lon contained m an abstract sutmvtted by the author(s). Contents of the paper, aspresented, have not~Bn reviewed by the .%eiety of Petroleum Engineers and are subjecf 10 correction by the author(s). The material, as presented does not necassarify reflect any posifion of the Scmety of Petroleum Engineers, its officers, or members Papers, presented at Petroleum &gineers ?%sswn !0 copy IS restricted 10 an abstract of "ot mom than 300 SPE meetln s are su c to publication rev!ew by Editorial Committees of the %ciely of words Illustrations may not be copied The abstract should contain conspicuous '1? ackncwle en?ant of where and by wfwm the paper IS presen{ed Write Librarian, SPE, P O Sox 8338 6, Richardson, TX 75063-3636, U.S.A fax 01-214.952.9435Abstract This paper discusses a number of aspects in relation to simulating unfiltered produced water re-injection (PWRI) under fracturing conditions.A numerical model has been developed that fully couples the reservoir engineering and tlacture mechanics aspects of the problem, and includes features such as finite, non-uniform fracture conductivity, fracture growth, filtercake build-up on the fracture face, formation impairment around the fracture, and backstresses resulting from pore pressure inflation and formation cooling.An essential difference with simulation of conventional waterflood fracturing is that owing to fracture fill-up with injected solids the fracture conductivity cannot be assumed infinite any more. This relates to the important PWRI issue of where the injected solids go. Using our model, we show that the pressure drop over a finite conductivity fracture can lead to a significant increase in fracture volume without necessarily leading to a significantly higher injection pressure. Thus, a picture emerges in which the fracture conductivity 'adjusts' itself in order to accommodate injected solids. This picture allows the computation of well injectivity as a function of total injected water volume, solids loading, etc. This concept can also be used to qualitatively explain the PWRI field observation that injectivity appears to be partially or fully reversible as a function of water quality, A field example from the Middle East is presented. The effect of parameters such as water quality, formation stiffness, and filtercake permeability on well injectivity and fracture size is discussed. It is shown that for most practical applications, an approximate analytical formulation for the computation of well injectivity and fracture size provides good results.

Application of New Fall-Off Test Interpretation Methodology to Fractured Water Injection Wells Offshore Sakhalin

“…The above low-stress contrasts do indeed result in an approximately radial fracture, as was verfied using simulations [12][13] , so the radial fracture shape appears justified in this case. The corresponding best-fit parameters for length shrinkage are: dimensionless fracture conductivity FCD = 625, fracture face skin S = 2.0, "speed of length shrinkage" parameter delpat = 0.15, and distance to 90º intersecting faults ≈1.5 km.…”

“…The existence of such boundaries is well known in this particular reservoir, which is largely compartmentalized. As a first step, the pressure change before fracture shrinkage can be matched using a radial fracture (h=2L) and a "typical" value for Young's modulus for a tight sandstone, E/(1-n2) » 15 GPa, one obtains a fracture length L = 25 m. This value is in line with waterflood fracture simulations using an in-house simulator [12][13] . As a next step, the complete pressure decline curve is matched using Eq.…”

“…These large fracture dimensions raised some concerns, as this may imply that the induced fracture had penetrated from the reservoir into an adjacent infinite aquifer, thereby significantly reducing the sweep. However, these large fracture dimensions were significantly above what was obtained from fracture simulations [12][13] . Fig.…”

“…In its most simple form, this can be achieved using the combined equations (11) and (12). Contrary to what was suggested previously 11 , Young's modulus cannot be determined from fall-off test analysis but should be provided as an input.…”

“…After each injector was fully on-stream, the practice of regular fall-off testing to monitor fracture growth continued. All fall-off tests were interpreted using the methodology outlined in section 2 of this paper, and the resulting fracture dimensions (length, height) were compared with predictions using an in-house simulator 12,13 . Below, we will present results of these fall-off tests and illustrate the problems / issues encountered using a few "typical" examples.…”