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As a fundamental technique to stimulate limestone formations, acid fracturing is now facing the problem of high fracture pressure. The fatigue damage in steel is a powerful strategy to reduce strength. And acidizing is the key step of acid fracturing and a useful tool to minimize limestone fracture pressure. Therefore, the combination of acid fracturing and fatigue treatment was investigated in this study. In addition, the interaction between acid fractures and calcite veins and weathering zones is still unclear. And the geometry of acid fractures in highly deviated wells hasn't been investigated in detail. Hence, there are three primary aims of this study: 1. To investigate the specific performance of fatigue acid fracturing in the limestone. 2. To ascertain the interaction between acid fractures and calcite veins and weathering zones. 3. To clarify the acid fractures morphology in highly deviated wells. Based on the tri-axial acid fracturing system, a sequence of conventional acid fracturing and fatigue acid fracturing experiments were carried out in vertical, highly deviated, and horizontal wells. The experimental results demonstrate that fatigue acid fracturing effectively reduced the fracture pressure and weakened stress shadow effect. And the inclination degree is an influencing factor of fracture morphology complexity. The weathering zone and the calcite blocks had a noticeable impact on fracture propagation process. According to the experimental results, the fatigue acid fracturing will generate fresh insight into limestone formation stimulation treatments.

Acid fracturing of carbonate rock starts with formation breakdown and fracture propagation caused by stimulation fluids. The effectiveness of acid fracturing largely depends on the behavior of fracture propagation. While fracture propagation of fractured carbonate rock is greatly influenced by natural fractures and fluid types. So far, some scholars have studied the influence of nonreactive fracturing fluid on fracture propagation, but the comparison of effect between nonreactive fracturing fluid and reactive fracturing fluid on fracture propagation is rarely noticed. In this paper, true tri-axial experiments on carbonate rock (200 mm × 200 mm × 200 mm) are presented; the experiment made a comparative study on the effect of nonreactive fracturing fluid (water, guar fracturing fluid) and reactive fracturing fluid (self-generated acid, gelled acid, VES acid) on fracture propagation in fractured carbonate rock. Besides, fracture morphology was revealed through CT scan and 3D reconstruction technology. In this way, influence of natural fractures and fluid types on hydraulic fracture propagation during acid fracturing was quantitatively analyzed. Results show that these two factors indeed exert great impact on fracture propagation behavior. As acid is more conducive to connection of natural fractures, hydraulic fractures induced by acid can propagate along natural fractures after initiating from natural fractures, and the process would not be restricted by maximum horizontal stress. As a result, a longitudinal fracture parallel to the wellbore is formed. Compared with self-generated acid and gelled acid, VES acid is more liable to connect natural fractures and form complex fracture network during acid fracturing. No matter reactive or nonreactive fracturing fluid is applied to conduct fracturing experiments, hydraulic fracture propagation in carbonate rock with few natural fractures would be restricted by maximum horizontal stress, and transverse fractures perpendicular to the wellbore would be formed after fracturing. In addition, breakdown pressure of rock samples is also affected by natural fractures and fluid types. To be specific, gelled acid decreases rock's breakdown pressure to the greatest extent when numerous natural fractures exist in the rock; the breakdown pressure caused by gelled acid is only 56.7% of which caused by water. However, the gelled acid has little contribution to decrease the breakdown pressure of the carbonate rock with few natural fractures. In a word, the experiment reveals effect of stimulation fluid types on hydraulic fracture propagation in fractured carbonate rock, which provides some guidance for acid-fracturing treatment in carbonate reservoir.

Acid etched fracture length and conductivity affect the carbonate stimulation effect. In this paper, acid-etched experiments with large-scale rock plate (1000 mm × 100 mm × 15 mm) were performed to investigate the acid etched fracture length, pattern, and conductivity. The relayed rock plates were etched with spent acid, collected from the outlet of previous plates, to simulate acid flow and reaction in a long artificial fracture. The rock plates were scanned with laser profilometer before and after acid etching to study etch behavior. The acid-etched fracture conductivity was measured under different closure stresses. The effective acid-etched fracture length is the fracture position with residual acid concentration. The results showed that the effective acid-etched fracture lengths with retarded acid were much higher than conventional acid, and most of the acid is consumed near the fracture heel. The acid-etched fracture pattern of self-diverting acid shows stronger non-uniformity with higher acid concentration, and more uniformly with lower acid concentration. The acid-etched fracture conductivity distribution along the whole artificial fracture is obtained with several relayed large-scale rock plates. And the acid-etched fracture conductivity obtained from large-scale rock plate is much lower than that from an API (American Petroleum Institute) rock plate. This study provided an experimental measure for acid-etched fracture length and global fracture conductivity evaluation.

On account of mud loss occurred during drilling in Dengying formation in Gaomo district of Sichuan Basin, factors affecting mud loss and optimization of acid system for acidizing in Mud-damaged deep fractured carbonated formation were studied. Carbonate core plugs with a diameter of 25.4 mm and a length of 50 mm were used as experimental samples to simulate natural fractures by artificially splitting, and experiments of mud loss in fractures varying from 10 μm to 250 μm were carried out. Results showed that fractures wider than 150 μm are main channels for mud loss. As fractures widen, the volume of mud loss increases while mud damages intensify first and then relieve; when the width is 150 μm, damages reach the top rate of 95.8%. When the fracture surface become rougher, the volume of mud loss decreases; in this case, the surface roughness has less impact on mud loss with widening of fractures. Then acidizing experiments of gelled acid, divert acid, and compound acid were carried out respectively in order to optimize acidizing system. Results are as follows: when fractures are narrower than 50 μm, acidizing has poor effect since all acid can hardly break through the mud damage zone to enter fractures; fractures at the width of 50 μm–200 μm enjoy better acidizing effect, and the fracture permeability recovery rate reaches 100%~400%. Since acidizing by gelled acid caused the most obvious wormholes that were thick and deep, gelled acid shows better acidizing effect than divert acid and even compound acid. As the width of fractures wider than 200 μm increased, wormholes parallel to the direction of oil and gas seepage changed into uniform etching that was wide and shallow, thus acidizing has poorer effect. Hence, selection of acid system for acidizing of mud-damaged deep fractured carbonate formation can take reference from this study.

Acid fracturing is a useful technique to stimulate carbonate reservoirs. Nowadays, acid leak-off becomes a vital problem during the operation of acid fracturing. In order to encounter this issue, alternating acid fracturing has been developed. However, the fracture propagation and the interaction between hydraulic fractures and natural fractures/caves during alternating acid fracturing are ambiguous. Hence, a series of alternating acid fracturing experiments covering coupling mechanics and chemical interaction were conducted for the first time. In this study, slick water, self-generating acid, cross-linked gel, and gelled acid were used as a fracturing fluid whereas the specimens for this experiment are carbonate outcrop which excavated from Dengsi Formation. The results show that: (1) acid fracturing can reduce fracturing pressure effectively as the acid will etch and weaken the natural fractures to let them easily connecting each other; (2) different pad fluid properties will have different impacts on the acid fracturing performance; (3) the interaction of hydraulic fractures and natural filled caves can be divided into five categories, and two of them had been observed in the experiments. Moreover, a new curvature-calculation algorithm has been developed based on the 3-D scanning technology. The results have a great significance for acid fracturing design and physical simulation experiments.