Stimulation of Carbonates Combining Acid Fracturing With Proppant (CAPF): A Revolutionary Approach for Enhancement of Sustained Fracture Conductivity and Effective Fracture Half-length

Abstract

The success of conventional fracturing (using non-reactive fluids to carry proppant) and acid fracturing is dependant on both the creation of effective fracture conductivity and fracture penetration (fracture half-length). With acid fracturing, nonuniform acid-etching (or differential etching) of the fracture face creates lasting conductivity as long as stable points of support (asperities) exist along the etched fracture length. These hold the channels open and connected to the wellbore following fracture mechanical closure. However, both field experience and laboratory work have shown that even fairly competent carbonates soften and creep under closure stresses after contact with acid, thus, potentially resulting in poor retention of acid-etched fracture conductivity. Preservation of fracture conductivity becomes even more challenging in case of high effective closure pressure. Furthermore, acid fracture conductivity is dependant on surface etching patterns, which are determined by uneven permeability and mineralogy distributions. Therefore, a very clean, homogeneous isotropic carbonate may not be a good candidate for acid fracturing since a fairly uniformly etched fracture might close completely at bottomhole producing pressures. Also, carbonate formations with more than approximately 30 percent insoluble components are generally not good candidates because overall acid-etched fracture conductivity may be impaired due to low solubility and also the release of insoluble materials may tend to plug any conductive etched patterns created by the acid. The effective length of the acid-etched fracture is limited by the distance the acid can travel along the fracture and adequately etch the fracture faces before becoming spent. When acid fracturing, the etched length, not the hydraulic length, is considered the effective fracture length. Effective acid penetration will most often be shorter than any proppant placement (due to often high and increasing leak-off rates with time, and high reaction rates, especially at elevated temperatures). An indeed rare, but in theory, powerful well stimulation technique is the combination of acid fracturing (i.e., creation of a hydraulic fracture using reactive acid fluid) with proppant (CAPF) to provide permanent conductivity. Unless proppant is squeezed into the acid fracture before the end of the job, the conductivity of an acid fracture is vulnerably retained pending the stability of asperities all along the height/length of the fracture. Thus, the desire to include proppant in fracture acidizing treatments is conspicuous (but not limited to) "clean" carbonates (exhibiting uniform mineralogy and permeability), carbonates at high effective stress conditions, "soft" carbonates of any permeability (excluding high porosity chalks), low temperature dolomites (with low reaction rates) and together with organic acids where small and vulnerable etched-fracture widths are prevalent. Also, intuitively, effective fracture half-length may be extended if acid (or non-reactive fluids) can transport proppant beyond the etched penetration length "all the way" to the hydraulic tip of the fracture or even extend the hydraulic length for typical short acid fractures. A methodology proposed by Dowell more than three decades ago "Maximum Conductivity Stimulation" (MCS) is probably the first discussion of the idea of combining acid with proppant fracturing. However, the idea did not establish roots in the oil and gas industry for reasons discussed in this paper. Clearly, one missing ingredient was the lack of today's state of- the-art modeling tools for determining suitable applications and procedures. This paper presents and uses a recently developed planar 3D, gridded, FEM (finite element method) multi-layer (with varying percent of limestone/dolomite including non-reactive layers) acid fracturing model. This model fully couples rock mechanics (fracture width and propagation), matrix and natural fracture fluid loss (and effects of acid and non-acid gel fluid stages to increase and reduce fluid loss, respectively), acid reaction/acid diffusion, fluid flow, and proppant/acid transport into a single solution. Such a capability is unique at this time, and, in general, only a 3D gridded model is capable of such simulations due to the complex interactions. Case histories are examined in this paper as possible targets for CAPF. The extraordinary simulation results from modeling of this combined process and its impact on well productivity are discussed.

Effect of Matrix Acidizing on The Performance of Selected Niger Delta Reservoirs

“…Acid treatment involves pumping highly pressurized acid into the well, thereby dissolving sediments to improve permeability. This process forms channels through which the hydrocarbons may flow [2]. The most common acid employed to stimulate production is Hydrochloric (HCL) which is useful in removing calcite materials from reservoirs and widely used in carbonate acidizing [3].…”

Assessment of oil-producing wells by means of stimulation approach through matrix acidizing: a case study in the Azraq region

“…Two major well-acidizing approaches are often applicable for curing damage around the wellbore, namely, matrix acidizing and acid fracturing (Civan 2007;Bale et al 2010;Morsy et al 2013;Liang et al 2017;Leung and Mahmud 2019). The matrix acidizing process is a chemical stimulation technique that involves the injection of acid mixtures below the fracture pressures of rocks near the wellbore.…”

“…Its purpose is dissolving sediments to reinforce rock permeability and the establishment of a clean zone within the reservoir, thus eliminating the restrictions for oil flux into the wellbore zone. Commonly, matrix acidizing is applied in case of tighter carbonate and sandstone formations; it is recommended in industry to utilize acids with mass concentrations, in the range of 10-15%, while acid fracturing is implemented in case of carbonate formations (Bale et al 2010;Li et al 2015;Liang et al 2017;Kolin et al and Kurevija et al 2018) with relatively higher acid mass concentrations, in the range of 20-30%. This method involves acid injection to create channels or so-called wormholes, as the resulting rock fractures, under pressure from the hydrocarbons, might immigrate through.…”

Matrix Acidizing Characteristics in Shale Formations

“…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]. The most common acid employed to stimulate production is hydrochloric (HCl), which is useful in removing carbonates from reservoirs.…”