Applications of Real-Time Matrix-Acidizing Evaluation Method

Abstract

Summary A new technique for real-time matrix-acidizing job evaluation has recently been presented that uses reservoir transients during acid injection to simulate pressure response of the unperturbed reservoir and compares simulated pressures with measured values. The difference between these values is attributed to the changing skin effect. An associated pretreatment injection/falloff test is presented as an essential clement of that method. It involves injection of an inert fluid (water or reservoir oil) and analysis of the ensuing falloff for a period during which pumping has stopped, just before acid enters the perforations. Injection resumes immediately thereafter. Two field case studies, a water-injection well in a fissured carbonate reservoir and an oil well completed in a sandstone reservoir, demonstrate the technique. For both treatments, conclusions are drawn regarding the effectiveness of the acid injection. The job evolution is demonstrated, and forecasts of expected well performance for unstimulated and stimulated cases are presented. The skin evolution is shown as a function of injection time. The simulated and measured pressure values for both treatments are shown, indicating the real-time job progression. Introduction Matrix stimulation is a treatment intended to remove near-wellbore damage. The decision whether to stimulate a well and the choice of treatment should depend on a comprehensive pretreatment analysis and on data gathering. Afterward, job effectiveness can be assessed through posttreatment evaluation. While this is true for hydraulic fracturing, where treatment cost and economics may justify pre- and posttreatment pressure-transient tests, it is not usually true for matrix treatments. Also, such a practice, when applied to matrix treatments, does not allow assessment of job advancement in real time, while the treatment is pumped. The concept of the skin effect has been used as a measure of near-wellbore flow impairment, but the total skin effect is a multicomponent quantity including mechanical skin effects. In fact, the skin effect lumps together any deviation from an ideal open hole, vertical well, and undamaged formation. "Damage" may be caused by a number of phenomena on which traditional acidizing has no effect. In gas wells, the skin effect is also rate-dependent. Finally, in saturated oil reservoirs or gas-condensate wells, skin arising from the appearance of an extra gas or oil phase has often been inferred. All of the above effects point toward the necessity of studying the well and its characteristics before any stimulation treatment is undertaken. For matrix stimulation, only that portion of the skin effect that results from formation damage can be removed chemically. If the skin effect owing to damage is quantified, then the treatment should reduce the total skin effect by that amount. Further, to be cost-effective, the injected volume and the pumping time should be minimized. Several attempts have been made to evaluate the effectiveness of a remedial matrix treatment by monitoring the evolution of the skin factor in real time. McLeod and Coulter used a technique in which each stage of injection or shut-in during the treatment is considered a short individual well test. The transient reservoir pressure response to fluid injection is analyzed and interpreted to obtain permeability values and damaged-zone radius. Analysis of pressure transients is valid only if the skin factor is not changing, however; hence, McLeod and Coulter's method must be considered a stepwise approximation. Furthermore, accurate real-time monitoring is not feasible. The method would require changes in job execution by shutting down and comparing falloffs throughout the pumping schedule each time an assessment of the skin effect is needed. Paccaloni was the first to introduce a method that uses instantaneous pressure and rates to compute the skin factor continuously at any given time during the treatment. The method, based on the steady-state, single-phase, radial version of Darcy's law, uses the concept of a finite-radius "acid bank." Experience indicates that this radius is usually assumed to be 3 to 4 ft [0.9 to 1.2 m]. Paccaloni used the concept of damage ratio as an assessment of the near-wellbore flow impairment, which can be simply defined as ............................(1) A series of damage-ratio curves for a range of injection rates and wellhead pressures is then plotted with known and assumed well and reservoir data. As the stimulation fluid is injected, the measured wellhead pressures and rates are plotted to indicate the treatment progress. The assumption of "steady state" could cause errors because transient behavior is in effect for a time that greatly exceeds injection time. In fact, the transient bottomhole pressure (BHP) would result in a pressure departure that would be misinterpreted as a skin effect: ............................(2) which would result in the injection of more acid than i, really needed. Further, unintentional changes in injection rate would also be construed as changes in the skin effect. Prouvost and Economides presented a new technique that allows a continuous calculation of the skin effect during the course of the treatment. This technique is based on a calculation of the pressure response of the reservoir to the same sandface rate history and the same sequence of fluids as in the actual treatment, but with a constant skin effect, sO. At any time, the difference between the simulated pressure response, p (t, s), and the value measured during the treatment, p (t), is interpreted as resulting from the instantaneous pressure arising from the skin. This changing difference is attributed, under a constant rate, to the changing skin effect, and for any rate. ........... (3) where s(t) and i(t) are the skin effect and the injection rate at time t, respectively, and so is the value of the skin effect used for simulation. so can be any constant value, but it is usually more meaningful to use either the initial skin value (before the acid treatment) or zero. Eq. 3 demonstrates that to avoid any misinterpretation of the difference [p (t)-p (t, s)], the simulation must accurately model the reservoir response under constant skin conditions. With the computation power now available at most wellsites, it is possible to compute and to display in real time the evolution of the simulated pressure response and therefore of the changing skin. Whenever the values of the well and reservoir parameters required for p (t, s) are not available (which is often the case for matrix treatments), an alternative procedure for obtaining them comprises a short injection/falloff test (discussed later). Fig. 1 illustrates pressure profiles that can be obtained with this method. In this case, the data set, a simulated case, illustrates the main features of the technique. The injection rate was slightly oscillating around 1 bbl/min [0.16 m /min] until 1.6 hours, when it suddenly dropped to 0.25 bbl/min [0.04 m /min]. SPEPE P. 401^