Small-Scale Fracture Conductivity Created by Modern Acid-Fracture Fluids

Abstract

The effects of acid solutions injected into hydraulic fractures created in carbonate formations can be assessed at the laboratory scale in acid fracture conductivity tests that mimic the conditions in an actual acid fracture treatment. We conducted a series of acid fracture conductivity tests using a protocol that mimics the fluxes in a hydraulic fracture, both in the main flow direction along the fracture, and in the fluid loss direction. In our tests, the injection rate into the fracture is much higher than in many previous tests, and the fluid loss flux is controlled to match field fluid loss rates. We studied three commonly used acid fracturing fluids---an acid viscosified with polymer, an emulsified acid system, and an acid viscosified with surfactants---at elevated temperatures of 200ºF and 275ºF. The acid fracture conductivity apparatus is similar to a standard API fracture conductivity cell, but with a capacity to hold core samples that are 3 in. long in the leakoff direction. The long cores allow for better control of leakoff as the acid creates wormholes into the core samples. In these tests, acid was pumped through the fracture for contact times ranging from 15 to 60 minutes. After the fracture surfaces were carefully characterized with a surface profilometer, the fracture conductivity was measured at increments of closure stress, up to a maximum closure stress of 6,000 psi. In this paper, we present the results obtained from a series of experiments with these fluids using Indiana limestone and dolomite core samples. Among the findings: Introduction Acid fracturing is a well stimulation process in which acid dissolution along the face of the hydraulically induced fracture is expected to create lasting conductivity after fracture closure. However, conductivity after fracture closure requires that the fracture face is non-uniformly etched by the acid while the strength of the rock is still maintained at high levels to withstand the closure stress. At low closure stress, the etched pattern of the fracture face should have a dominant influence on the resulting fracture conductivity as long as the strength of the rock can withstand the load. As the closure stress is increased, surface features along the fracture faces may be crushed and the fracture conductivity is more dependent on the rock strength than on the initial etching pattern. The success of the acid fracturing process depends highly on the resulting fracture conductivity which is very difficult to predict because it inherently depends on a stochastic process and is affected by a wide range of parameters. Most predictions of conductivity are made with the empirical correlation developed by Nierode and Kruk.1 This correlation was based on experiments using 1 inch diameter by 2 to 3 inch long fractured cores, with no fluid loss through the rock samples. The acid fluxes through the fracture in these experiments were much lower than expected in a field fracture. While there have been several other experimental studies done2–5, they were either done at conditions that did not scale to field conditions or did not consider rock weakening or the etching pattern on resulting fracture conductivity while the effect of only a few parameters were studied in a limited number of experiments.

Large-Scale Acid Fracturing Based on a Large-Scale Conductivity Apparatus

“…To deeply investigate the relationship between acid etching and conductivity in large-scale experiments, the results of this study were compared with the findings of other small-scale experiments. ,− The physical and scanned morphologies in small-scale experiments had deeper channels in the fracture where an initial acid–rock reaction occurred compared with the other locations. The etched grooves were more obvious in the vicinity of the inlet.…”

“…Different conductivity results in previous studies were selected for comparison: #1 comprised SET2 4 and SET3 7 cores used by Malagon, #2 comprised the samples by Pournik, and #3 comprised the Indiana limestone data from Hill . All the data were from the experiments on gelled acid etching under similar experimental conditions.…”

“…The difference and heterogeneity of cores were considered in another group of small cores (set E), which were subjected to acid etching and conductivity experiments for a comparison. The experiments were conducted at 120 °C in 20% gelled acid, and the outflow parameter and reaction time were the same as mentioned in Malagon . The morphologies and conductivity are shown in Figures and .…”

“…Besides, the results represented the conductivity of near-well-bore areas, instead of the whole fracture, as the influenced inlet area had a portion of the core and was short. Moreover, the test result was an inappropriate representative of the fracture conductivity of the carbonate more than 100 m underground. , Similarly, small-scale rock cores could not fully reflect their heterogeneity and the heterogeneity affected the morphology of the fracture surfaces, resulting in an inappropriate morphology of the carbonate reservoir where both were related to each other.…”

A Novel Pore-Scale Thermal-Fracture-Acidizing Model With Heterogeneous Rock Properties

“…They also found that emulsified acid led to greater performance than in-situ-gelled acid. Rahim et al (2002) presented a general overview of how fracture acidizing is being conducted in the pre-Khuff and Khuff formations of Ghawar field, Saudi Arabia. They described how the Khuff formation has been an ideal candidate for the previous 50 fracture-acidizing treatments because of the heterogeneous nature of the formation (leading to high fracture conductivity).…”