Summary A fracture-acidizing treatment of carbonate formations can be considered successful when a relatively good fracture conductivity remains after treatment. To reach such a goal, an uneven etching of the fracture by acid is expected, so that channels are created that maintain the fracture hydraulically open even after "mechanical" closure, and therefore enhance productivity. Residual conductivity is the consequence of uneven etching of the surface, but the way this etching occurs in the field is not well understood and therefore poorly described. We thus propose in this paper an experimental method aiming at defining a methodology to investigate and quantitatively characterize how acid injection conditions affect the fracture surfaces, and how fracture conductivity can be estimated from the so-created surface topography. The statistical investigation of surface topography associated with acidizing experiments proposed in this paper, offers the possibility to evaluate the best fluid formulation and flow rate for a given formation type under well conditions. The field application of this method is evident, since it provides a new and interesting tool for selecting fluid and forecasting the behavior of the fracture after an acid fracturing job. Introduction Acid fracturing is a classical treatment used in carbonate formations to improve well productivity. To reach that aim, acid is injected either at a pressure sufficient to fracture the formation or into an already hydraulically induced fracture. As acid flows along the fracture, it dissolves portions of the fracture faces, generally in a non-uniform manner, so that conductive channels are created that remain "hydraulically" open even after "mechanical" closure. The treatment is thus as successful as the so created fracture is long and conductive. The efficient length of the fracture in a given formation is determined by injection conditions (flow rate), injection fluid composition, acid-formation reactivity (acid spending) and acid fluid loss (or leakoff) from the fracture into the formation. On the other hand, the mechanisms and the conditions that give a conductive acid fracture are poorly evidenced and described in the literature. Acid composition and fluid injection sequences are essential parameters in the design of an acid fracturing treatment. The two jobs described in Table 1, performed on two wells on the ABK field, provide clear evidence of the influence of acid formulation and alternating stages on residual fracture conductivity. Though these treatments were performed in nearly the same formations (dolomites). evaluation of post treatment performance displays (Table 1) a better efficiency of the first job. 61% of the injected acid indeed contributed to the etching, whereas for the second job, the major part of the injected volume was lost in the formation because of leakoff effects and consecutive wormholes formation, leading to a worse residual fracture conductivity. Leakoff by decreasing the acid available for the etching of the fracture faces, reduces the treatment efficiency. The need to control acid fluid loss, and the consecutive formation of channels perpendicular to the main flow (wormholes) led to many studies aiming at identifying the mechanisms of wormhole creation. These studies show that depending on injection rate, three different wormholing mechanisms are identified. Though fluid losses can not be avoided in acid fracturing, these studies enable a better understanding of the way acid is consumed through leakoff depending on the fluid rate, and therefore the way they can be reduced. The third important factor affecting fracture geometry is acid spending during injection. Acid spending is mainly controlled by the acid - rock reactivity, that in turns depends on many factors such as injection conditions, acid concentration, composition, formation composition, temperature, fracture width. On the field, success of an acid fracturing job is evaluated from post treatment performances.

Summary In practice, gel formulations used in water shutoff and conformance improvement treatments are subjected to hours of strong shearing in surface and well equipment (i.e., pumps, tubings, etc.) and in the near-wellbore part of the formation before being left under quiescent conditions (well shut-in). The purpose of rheological measurements presented in this paper is to assess the influence on polyacrylamide/chromium (III) gelation of shearing sequences representative of those occurring in the field. The gel properties investigated are the gelation time and final yield stress, which are related, respectively, to the pumping time and to the maximum differential pressure the gel can withstand in the reservoir (matrix or fracture). Two different polyacrylamide/chromium (III) formulations were analyzed, one of which, involving a high-molecular-weight polyacrylamide, is adapted for plugging fractures, and the other one, involving a low-molecular-weight polyacrylamide, is usually employed for plugging porous matrices. These formulations had previously been used in some specific field applications. The experiments consisted of a series of viscosity, storage modulus and yield stress measurements on the gelling solutions, with the purpose of quantitatively characterizing the gelation kinetics and final strength of the gel formed. The results demonstrate that the effect of shearing is to restrain gelation and, in particular, to limit the viscosity rise due to the formation of aggregates of crosslinked polymers (microgels). The gelation time, defined in relation to this viscosity rise, increases with shear rate for the high-molecular-weight (non-Newtonian) formulation, but is unaffected by shear for the low-molecular-weight (Newtonian) formulation. Experiments have also been conducted in which an extended initial period of strong shearing was followed by another extended period of rest. The final gel strength of both formulations, evaluated from yield stress measurements, turned out to be independent of the shear rate history. Introduction Polymer/crosslinker formulations are increasingly used as permeability modifiers in water shutoff or conformance improvement treatments.1–3 The formulations are usually selected from bottle tests, in which polymer and crosslinker solutions prepared under various conditions (of concentrations, brine salinity, pH, etc.) are mixed and the development of the gel is observed visually at reservoir temperature and described qualitatively in terms of a gelation time and some characteristics of the gel strength or firmness.4 The gelation time must be long enough to allow for placement of a sufficient gel volume, while the gel must be strong enough to withstand the pressure gradients expected in the formation. More quantitative methods for evaluating the gelation time of a given formulation consist of monitoring the evolution of the storage modulus at some low frequency or the viscosity at some low shear rate.4,5 On the other hand, there is no accepted method for quantitatively determining the final gel strength. Here, we propose to gauge the final gel strength by measuring its yield stress, which is the maximum stress the gel can withstand without flowing. This yield stress should be related to the ability of the gel to withstand pressure gradients in fractures and porous matrices, as demonstrated in the Appendix from a simple flow model. The work reported in this paper consists of a series of viscosity, storage modulus, and yield stress measurements carried out with two different polyacrylamide/chromium (III) acetate formulations. The purpose of these measurements is to quantitatively characterize the gelation kinetics and final gel strength of gels formed under various shearing conditions. It is noted here that laboratory screening tests are in general carried out under quiescent conditions, while in reality the formulation is submitted to important shearing as it flows from the mixing pumps through the lines, then into the wellbore, and finally into the porous formation. The field data reported in Ref. 6 can be used to evaluate typical shearing rates and times. First, the polymer and crosslinker solutions are mixed for 10 minutes by pump recirculation, with an estimated average shear rate of 1,000 seconds–1. Then, the formulation is injected into the well at a rate of 10 m3/h (1 BPM) by using either a Macaroni with an internal diameter of 1.52 in. and a length of 2370 m, or through a 2 7/8-in. tubing (internal diameter 2.4 in.) and the same length, thus experiencing wall shear rates in the range of 100-500 seconds–1 during 20 to 40 minutes. In the formation, the pore wall shear rate, estimated within the capillary bundle approximation for the porous medium, reaches values of the order of 4,000 seconds1–1 at the sandface, then decreases like the inverse of the penetration depth in the formation. After a pumping time of 2 hours, this depth is around 2 m, giving a shear rate of 200 seconds–1. Once the formulation has been installed in the formation, that is to say, after a few hours of shearing, it is allowed to rest for a while (well shut-in) before well production is resumed. It is not clear whether the strong shearing experienced by the gel solution during the placement has an effect on the gelation time and final gel strength and whether these parameters can be safely determined from experiments carried out in the laboratory on quiescent samples. There have been many studies of the effect of an applied shear rate on the gelation of polymer/crosslinker systems, most of them being concerned with xanthan-Cr (III) systems. These studies have shown that shear strongly affects the gelation process, both in bulk and in-situ (i.e., in porous media). Bulk measurements have been carried out by steady and oscillatory shear,7–12 by pulse shearometry,8 and, more recently, by superimposing steady shear to small-amplitude oscillatory flow,12 the latter method allowing one to monitor the evolution of the viscosity (at the applied shear rate) simultaneously with the storage modulus (at a chosen frequency). Experiments in porous media have mostly consisted in measuring the pressure drop across subsections of the porous medium for constant-flow-rate injections8–10 and in recording the polymer and/or crosslinker effluent profiles at the core outlet.8,9 The "re-formability" or "re-healing" of samples initially subjected to prolonged periods of shear and then left under quiescent conditions (thus mimicking the field process) has also been examined, both in bulk and in porous media.7–12

This paper was prepared for presentation at the 1999 SPE International Symposium on Oilfield Chemistry held in Houston, Texas, 16-19 February 1999.

This paper gives some recommendations of the pertinent rheological measurements for an efficient gel screening in water shut off and conformance improvement treatments. The proposed methodology is based on measurements of gelation time and final yield stress, which are related, respectively, to the pumping time and to the maximum differential pressure the gel can withstand in the reservoir (matrix or fracture). A laboratory study has been carried out with two polyacrylamide/chromium (III) formulations, one of which, involving a high-molecular-weight polyacrylamide, is adapted for plugging fractures, and the other one, involving a lowmolecular-weight polyacrylamide, is usually employed for plugging porous matrices. These formulations have been used in some specific field applications. A series of viscosity, storage modulus and yield stress measurements on the gelling solutions are presented in this paper, with the purpose of quantitatively characterizing the gelation kinetics and final strength of the gel formed. The proposed methodology is utilized for investigating in detail the influence of shearing on the gelation time and final gel strength. In practice, the gel formulation is in fact subjected to hours of strong shearing in the surface facilities (i.e., pumps, tubings,...) and in the nearwellbore part of the formation before being left under quiescent conditions (well shut-in).The results demonstrate that shearing has for effect to restrain gelation and, in particular, to limit the viscosity rise due to the formation of aggregates of crosslinked polymers (microgels). The gelation time, defined in relation to this viscosity rise, strongly increases with shear rate for the high-molecularweight (non-Newtonian) formulation, but is unaffected by shear for the low-molecular weight (Newtonian) formulation. Experiments have also been conducted in which an extended initial period of strong shearing was followed by another extended period of rest. The final gel strength of both formulations, evaluated from yield stress measurements, turned out to be independent of the shear rate history.

This paper was prepared for presentation at the 8th Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, U.A.E., 11-14 October 1998.