Abstract
Matrix acidizing is typically used to remove drilling and completion damage to reservoir conductivity around the wellbore and dissolve calcite in natural fractures. Despite being a common procedure, few studies have investigated the effect of matrix acidizing on the physical properties and oil recovery factors in shales. This paper describes the effect of HCl acid on porosity, spontaneous imbibition, mechanical properties, and crack distribution in samples from the Eagle Ford, Mancos, Barnett and Marcellus shale formations. Some of the samples were completely immersed in different HCl solutions (1-3 wt%) at 93°C. We measured the porosity in both the acid-treated and non-treated samples. The treated and non-treated samples were then exposed to spontaneous water imbibition experiments to measure the improvement in oil recovery in both parallel and perpendicular to bedding planes. The mechanical properties of the acid-treated and non-treated samples were also measured in both parallel and perpendicular to bedding planes using the same acid concentrations. The samples were 2.54 and 3.81 cm in diameter and 2.54 to 5.08 cm in length. The measured porosities were 1-3% for the non-treated samples and 1.3-10.5% for the treated samples. We observed that the oil recovery factors of the spontaneous imbibition for the samples treated with acid were 47% from Eagle Ford, 53% from Mancos, 28% from Barnett, and 38% from Marcellus. The recovery factors from the non-treated samples were 12% from Eagle Ford, 4% from Mancos, 13% from Barnett, and 3% from Marcellus. Furthermore, we observed that spontaneous imbibition parallel to bedding planes is higher than imbibition in perpendicular to bedding planes direction, especially for Marcellus samples where the recovery factors varied from 4% for the samples drilled parallel to bedding planes to 38% for the samples drilled in perpendicular to bedding planes. Eagle Ford and Mancos samples showed a reduction in confining compressive strength ranging from 50- 60% when exposed to 3 wt% HCl solutions with more reduction in the samples drilled parallel to bedding planes.
Highlights
Acidizing treatments are commonly used to remove near wellbore damage and create artificial flow channels in carbonate formations, while limited treatments were done on shale rocks [1]
Shale formations may have highly variable mineralogies, which makes it difficult to predict the consequences of matrix acidizing
A limited number of studies have quantified the effect of HCl on physical properties of shale formations [4] and almost no information is known about its effect on shale oil recovery
Summary
Acidizing treatments are commonly used to remove near wellbore damage and create artificial flow channels in carbonate formations, while limited treatments were done on shale rocks [1]. It is important to consider damage mechanisms when designing a matrix treatment, as dissolving calcite, quartz, or clay minerals may affect the reservoirs differently [2]. Acid fracturing involves pumping highly pressurized acid into the well, physically fracturing the reservoir rock and dissolving sediments to improve permeability. This process forms channels through which the hydrocarbons may flow (Figures 1-10) [5]. A new application of combining the benefits of acidizing and propped hydraulic fracturing in unconventional shale formation showed a great improvement in gas production of Woodford shale formation, as acid is injected as a pre-flush treatment, and is used in different sub-stages of the hydraulic-fracturing process away from wellbore [6].
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