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 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.