An Environment Friendly Approach to Reduce the Breakdown Pressure of High Strength Unconventional Rocks by Cyclic Hydraulic Fracturing

Abstract

Unconventional hydrocarbon resources mostly found in highly stressed, overpressured, and deep formations, where the rock strength and integrity are very high. When fracturing these kinds of rocks, the hydraulic fracturing operation becomes much more challenging and difficult and in some cases reaches to the maximum pumping capacity limits without generating any fracture. This reduces the operational gap to optimally place the hydraulic fractures. Current stimulation methods to reduce the fracture pressures involvement with adverse environmental effects and high costs due to the entailment of water mixed with huge volumes of chemicals. In this study, a new environment friendly approach to reduce the breakdown pressure of the unconventional rock is presented. The new method incorporates the injection of chemical-free fracturing fluid in a series of cycles with a progressive increase of the pressurization rate in each cycle. This study is carried out on different cement blocks with varying petrophysical and mechanical properties to simulate real rock types. The results showed that the new method of cyclic fracturing can reduce the breakdown pressure to 24.6% in ultra-tight rocks, 19% in tight rocks, and 14.8% in medium- to low-permeability rocks. This reduction in breakdown pressure helped to overcome the operational challenges in the field and makes the fracturing operation much greener.

Relaxation damage control via fatigue-hydraulic fracturing in granitic rock as inferred from laboratory-, mine-, and field-scale experiments

“…For Tennessee sandstone, the decrease in the breakdown pressure caused by cyclic fracturing can largely be attributed to the reduction in tensile strength due to water saturation 53 . In ultra-tight concrete, the reduction percentage is higher (25%) compared with in low-and medium-strength cement blocks (15%) 55 . This finding on concrete is in line with experiments on Xujiahe sandstone 56 .…”

“…In general, the benefit reported is threefold: First, hydraulicfatigue testing allows the breakdown pressure to be lowered. Laboratory tests of cyclic hydraulic fracturing on Tennessee sandstone 53 , Pocheon granite 49 , Fontainebleau sandstone 54 , and cement-core plugs with variable strength properties 55 have been reported. The percentage of the reported reduction of breakdown pressure varies for different rock types and with the porosity and strength of individual rock types.…”

Cyclic Injection to Enhance Hydraulic Fracturing Efficiency: Insights from Laboratory Experiments

“…Kang et al [12] Tight Xujiahe sandstone Kang et al [12] Normal Xujiahe sandstone Patel et al [4] Tennessee sandstone Goyal et al [5] Tennessee sandstone Zhuang et al [16,17] Pocheon granite Diaz et al [18,19] Pocheon granite Stephansson et al [20] Pocheon granite Liang et al [21] Concrete Tariq et al [8] Concrete 6 Geofluids where b and σ f ′ are empirically fitted parameters, which are called the fatigue strength exponent and the fatigue strength, respectively. Figure 12 illustrates the concept of the σ a versus N f curve.…”

“…Several research groups have experimentally investigated the effect of cyclic loading on breakdown pressure reduction: Patel et al [4] and Goyal et al [5] conducted cyclic injection on sandstone; Zhuang et al [6] and Zang et al [7] conducted cyclic injection on granite; and Tariq et al [8] conducted cyclic injection on concrete. They reported that at laboratory scale, compared to conventional injection in which the injection pressure increases monotonically, the cyclic injection can reduce the breakdown pressure of sandstone [4,5], granite [6,7], and concrete [8]. For example, Zhuang et al [6] conducted cyclic injection tests on 34 Pucheon granite specimens, and they reported that cyclic injection can reduce the breakdown pressure for granitic rocks by at least 10%.…”

“…Patel et al [4] conducted cyclic injection tests on both dry and saturated Tennessee sandstones; they stated that for dry sandstone, cyclic injection can reduce the breakdown pressure by 15%. Tariq et al [8] conducted cyclic injection tests on concrete specimens with three different strengths. They observed that the effect of cyclic injection is more important on stronger concrete samples, and for strong concrete samples, cyclic injection can reduce the breakdown pressure by 20%.…”