Fracture Conductivity Created by Proppants and Acid in the Austin Chalk Formation

Abstract

The Austin Chalk formation has seen numerous periods of active development since the early 1980’s and has recently become a new focus as an unconventional resource. Due to the low permeability, fracturing, with proppant or acid, is required to economically produce the reservoir. This study presents the results of a series of conductivity tests performed to evaluate the stimulation efficiency of proppant fracturing and acid fracturing in the Austin Chalk. The study uses both downhole core and outcrop samples. The downhole cores were from an Austin Chalk well in Webb County, Texas. The outcrop samples were collected from similar Austin chalk sub-intervals that the core samples were from and were collected from a road cut in the Del Rio, Texas area. Testing samples were prepared for the fracture conductivity tests based on API RP-61. Fracture surfaces were created either by tensile fracturing the sample, or by saw-cutting. For acid fracture conductivity, 15% HCl was injected to create acid-etched surfaces, and the effect of acid contact time was investigated. For propped fracturing, proppant size and concentration were set as study parameters. The fracture surfaces were scanned before and after each experiment in order to characterize changes in the fracture surfaces, such as acid etching and proppant embedment. Proppant size distribution was measured by sieve tests before and after conductivity measurement to evaluate proppant crushing. Closure stress up to 6000 psi was applied during the conductivity measurements, and some experiments stopped before reaching this upper limit because the sample lost mechanical integrity under a lower closure stress. The study results show that because the mechanism of creating conductivity is different for acid fracturing and propped fracturing, the conductivities from the two stimulation methods behave differently. The initial conductivity varies but the level of conductivity at low closure stress generated from acid and proppant are similar. The conductivity from propped fracture exhibits a semi-log decline trend, with a low decline rate and sustained conductivity at high closure stress. The conductivity from the acid fracture experiments strongly depends on acid injection conditions and mechanical integrity of the rock after acidizing, indicating that design of the stimulation is critical for a successful treatment. Acid fracture conductivity in general declined more rapidly with increase in closure stress than did proppant conductivity. The observations from the experimental study revealed that both acidized and propped fractures have potential to create sufficient conductivities in the low permeability Austin Chalk formation.

Experimental and Modeling Investigation on Artificial Fracture Conductivity for Hybrid Fracturing in a Deep Carbonate Reservoir Using an Acid and a Proppant

“…Based on the reservoir parameters and the average acid injection parameters of prepad acid fracturing in Block S, the acid-fracturing model incorporating the heat exchange between the fracturing fluid and in the wellbore and fractures was adopted to determine the acid concentration and temperature during reaction at different positions along the hydraulic fracture length. 38 The key parameters inputted into the model calculation consisted of: an initial formation temperature of 180 °C and a prefracturing fluid volume of 400m 3 , followed by an acid fluid volume of 400m 3 . Additionally, both the fracturing fluid and acid were injected with a displacement of 8.0 m 3 /min.…”

“…Acid fracturing is one of the most effective technical methods for stimulating a carbonate hydrocarbon reservoir. ,, The nonuniform acid-etched morphology allows artificial fractures to maintain certain conductivity withstand high closure pressure, thereby enhancing well productivity. However, as closure pressure increases, the conductivity of acid-etched fractures rapidly declines, leading to a significant decrease in well productivity. , The rapid decline in fracture conductivity may occur in carbonate reservoirs under the following conditions: first, in ultradeep carbonate reservoirs due to high closure stress; second, the complex artificial fracture formations caused by diverting acid-fracturing, as narrow initial acid-etched width and limited conductivity result from the acid diversion caused by temporary plugging; , third, in uniformly acid-etched fracture where the fracture surface lacks sufficient raised support points to withstand closing pressure; fourth, in soft rock mechanical strength reservoirs where acid-fractures easily close . Hybrid acid-proppant fracturing treatment provides an effective method to prevent fracture support surface failure and seepage space narrow under high closure pressure by introducing proppants into nonuniformly acid-etched fractures …”

“…Acid fracturing is one of the most effective technical methods for stimulating a carbonate hydrocarbon reservoir. 1,2,3 The nonuniform acid-etched morphology allows artificial fractures to maintain certain conductivity withstand high closure pressure, thereby enhancing well productivity. However, as closure pressure increases, the conductivity of acid-etched fractures rapidly declines, leading to a significant decrease in well productivity.…”