Sequenced Fracture Degradable Diverters Improve Efficiency of Acid Fracturing in Multiple Perforated Intervals Completion Assembly

Abstract

Gas-bearing carbonate reservoirs in moderate to low permeability reservoirs have been targets for acid fracturing treatments in the Middle East. These formations typically exhibit high temperatures, medium to low porosity, and high heterogeneity in terms of lithology and reservoir properties. The heterogeneity dictates completion strategy, with multiple perforated intervals across large gross height in vertical wells with subsequent acid fracturing treatments that aim to cover all perforated intervals in a single treatment. But due to differences in lithology, intervals with high dolomite content are less likely to receive stimulation due to higher stress and reduced acid reactivity. Temperature logs performed on many wells after conventional acid fracturing treatments showed that these perforated intervals accept only a small amount of treating fluids, compared to intervals perforated in clean limestone. An efficient, non-damaging, near-wellbore diverter is required to efficient treat all intervals and improve productivity in such wells. The objective is to stimulate all existed intervals in a single pumping operation, regardless of reservoir heterogeneity, by using degradable diverting materials to temporarily isolate created fractures and redirect the flow to untreated areas. The diversion material used is a composite pill comprising a proprietary blend of degradable fibers and multimodal particles, designed to provide an effective isolation plug at the face of the reservoir in a consistent manner. Fibers are added to ensure the integrity of the diversion pills during delivery and to enhance the bridging mechanism. The use of fibers allows minimizing required diverter volume to few barrels and engineered multimodal diverting materials allow having very strong diversion pressure with small amount of the material. The process increases operational efficiency, well productivity, and estimated ultimate recovery. The materials used to provide temporary isolation have proprietary formulation that degrades within hours or days, depending on bottomhole temperature, with no need of intervention or pumping chemicals to break down the system. Two pilot treatments with degradable diverter were conducted in high temperature high pressure carbonate reservoirs. Extensive measures were undertaken to evaluate the treatments, including pressure analysis, separator tests, temperature logs, production log (PLT), pressure build up (PBU), and nodal analysis. Overall, the measurents and analysis of the treatments proved the efficiency of the degradable diverter for vertical wells: sharp pressure increase up to 1,600 psi when pills arrived at perforation; cooldown effects in all intervals on the post-fracturing temperature logs ensuring uniform distribution of the acid; high flowback gas rates, substantially higher than those of offset wells treated without the diverter; fracture response and signature observed on PBU data; PLT contribution from most of the perforated intervals confirming that treatments penetrated all intervals of interest; and nodal analysis with good production match showed long etched fracture half-length - a preferred fracture geometry for tight reservoirs.

Impact of Diversion on Acid Fracturing of Laminated Carbonate Formations: A Modeling Perspective

“…To validate the model, it was compared to an acid fracture job performed on a laminated formation. Rahim et al provided a temperature profile after an acid-fracturing job in a Middle Eastern laminated formation. They showed the lithology distribution where a calcite layer existed in the middle of a dolomite pay zone.…”

“…Carbonate formations are normally heterogeneous and in many cases contain both calcite and dolomite minerals that are sequentially laminated . Because of differences in reactivity and stress, acid preferentially etches the calcite layers and leaves the dolomite layers understimulated.…”

Development of Acid Fracturing Model for Naturally Fractured Reservoirs

“…There are multiple publications on improving the acid fracturing efficiency in naturally fractured reservoirs, many of which focus on using viscoelastic acid systems with diverters to reduce the acid loss form the main fractures by bridging the natural fractures (Rahim et al 2017;Williams et al 2016;McCartney et al 2017). However, the effect of natural fractures on the efficiency of the acid fracturing treatment and created fracture conductivity is not fully understood.…”

“…Gdanski (1993) studied experimentally the fluid loss control during acid fracturing and concluded that acid leakoff can be significantly reduced by using solids for plugging and bridging wormholes. Rahim et al (2017) presented results for successful application of degradable multi-modal particulate diverters mixed with fibers to enhance the leakoff control in near wellbore region and to improve the efficiency of acid fracturing treatment in heterogeneous formation. Williams et al (2016) conducted an experimental study on improving the acid leakoff control by using far-field solid particulate diverters which effectively bridge wormholes and hairline fractures away from the wellbore region.…”