This work presents the development of a methodology to numerically represent the acid treatment in a test plug, as well as to reproduce the different existing dissolution patterns and to obtain the corresponding values of pore volumes to breakthrough (PVBT). The numerical simulation is performed in a commercial CFD package that uses finite volume method. The modeling includes the effect of heterogeneous porosity/permeability and the presence of different types of minerals that impact the PVBT value, since they have different reaction rates at usual operation temperatures. Through these considerations, the formation of preferential channels, which are characteristics of the various patterns of wormhole, is captured by the numerical simulation.The goal of this development is the extraction of the characteristic PVBT curves for any pair formation/acid by numerical simulation. It is possible through the use of measured data during drilling, such as average porosity and range of variation, rock mineralogy, etc., and through the knowledge of reaction rates for each pair formation/acid. Using these data, the simulation is able to extract PVBT curves for different numerical test plugs, making it possible to prepare a statistical analysis that has greater significance than just a few experimental tests.The results show that PVBT curves obtained numerically are in good agreement with the physical behavior expected when compared to experiments. The variation range of the heterogeneous porosity and the presence of different minerals, which have distinct reactivity with acid, significantly change the behavior of the process for the same operating condition. Better understanding of acid treatment in carbonates (both limestone and dolomite) is important since the new Brazilian petroleum reservoirs are located below pre-salt layer. These rock formations are commonly subjected to acid stimulation in order to increase reservoir productivity. Therefore, the numerical PVBT curves obtained from this work could be used in simpler models to simulate the acid treatment in a reservoir scale. The use of more accurate curves can help the engineers to improve the design of operation conditions and, thus, increase the production capacity and distribute uniformly the treatment.
The deep and hard limestones found off-shore Brazil are being extensively evaluated, and some of them would require stimulation to enable commercial development. The main objective of this paper is to discuss the feasibility of acid fracturing stimulation in hard carbonates. Laboratory test are performed to verify whether an acid-induced fracture can withstand the high effective normal stresses expected during the exploitation stage of pre-salt development, keeping an acceptable conductivity. There is no agreement in the technical community regarding the survival of an acid fracture at closure stress greater than 5,000 psi. However, the experience that leads to this controversy is mainly based on: (1) wells on land and, (2) few published tests on rocks with mechanical properties compatible with those depths. An experimental program has been designed to evaluate these chances of acid fracture survival. A few initial essays with samples of these carbonate rocks have shown significantly higher rock strength than those commonly found in the literature. Initially, the Nierode & Kruk correlation (1973) is used to predict fracture conductivity behavior. Both mechanical strength and dissolved rock equivalent conductivity are measured from outcrops, and from soft and hard offshore carbonates samples. This work reports the results with outcrops and reservoir samples, in order to assess whether the adopted experimental methodology is consistent and reproducible. Some topics about acid fracture conductivity prediction are also addressed. Further research should be performed with pre-salt rock samples using the same procedures. IntroductionOpen and conductive acid fractures are difficult to obtain. The existence of such fracture depends on the resistance of their asperities, ridges, hills and valleys to avoid being overwhelmed by the rock that surrounds it. Could the Argonauts feat be replicated and an acid fracture is maintained always open? According to myth, the passage of the Argonauts on Argó through the Symplegades, the clashing rocks, anchored and kept these rocks forever open in the Bosphorus strait. The analogy with an acid fracture is direct. Unfortunately maintain an acid fracture always open is also a myth. Sometimes, acid fractures close.
Nowadays, it is commonplace to say that acid fracture conductivity depends on the fracture face asperities. Does it really depend on it? Almost thirty years ago, someone wrote, "We believe the conductivity measured in these tests is mainly due to the smoothing of peaks and valleys on the rough fracture faces, and is independent of rock heterogeneities due to the small sample size." Moreover, almost one year ago, one wrote, "More asperities touch and deform as the closure stress increases. The channels become even shorter and fewer openings are left." Between these two extremes, the asperities came to be pointed out as an essential factor to generate acid conductivity. Many published results from small, wet sawed and leveled carbonate rock samples support such claim. We did the same. Our experimental investigation on small scale carbonate samples with sawn faces, both from outcrops and well cores, reconfirm the existence of three main acid patterns namely uniform, channels and roughness. The design of experimental apparatus prevented that those etching patterns were artifact patterns. The acid etching patterns determine different conductivity behavior under confining stress. However, hydraulic fractures are tensile fractures and they are naturally rough. In nature, there is no such thing as a leveled fracture face. Tensile fracture faces could be rougher than fracture face after acid reaction. In fact, the first experimental results show that after acid reaction, linear roughness of tensile fractures can be larger, equal or less than linear roughness before acid reaction. This paper presents experimental results and discusses the asperities paradigm.
The deep and hard carbonates found offshore Brazil are being extensively evaluated, and some of them would require fracture stimulation to enable commercial development. Due to the high effective normal stresses expected during the exploitation stage of pre-salt development, acid fractures could have very low conductivity values, depending on formation mechanical properties. Since perform propped fractures is more risky, it is important to create selection criteria to know precisely the fracture technique to be used. There is no agreement in the technical community regarding the survival of an acid fracture at closure stress greater than 5,000 psi. However, the experience that leads to this controversy is mainly based on: (1) wells on land and, (2) few published tests on rocks with mechanical properties compatible with those depths. This article aims to discuss Petrobras’ fracturing selection criteria development. The experimental data currently available for acid fracturing Brazilian deep carbonates will be presented. Laboratory test are performed to verify whether an acid induced fracture can withstand high effective normal stresses, keeping an acceptable conductivity. Key mechanical properties are measured in order to understand acid fracture conductivity behavior under stress. These mechanical properties are correlated with log data to allow the use of laboratory results on the fracture technique decision of a particular well. Field data regarding acid fractures wells will also be addressed.
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