The dilemma of soaking a hydraulically fractured horizontal shale well prior to flowback – A decade literature review

Shale reservoirs are extensively exploited using hydraulic fracturing, which forms multiple cracks that connect with the existing natural fractures to create a continuous path for the gas stored in the kerogen to flow to the production well. Apart from the tedious nature of hydraulic fracturing, the mechanism of the storage and flow of gas is equally complex since multiple phases and scales are involved. An accurate understanding of hydraulic fracturing coupled with a strategy of analyzing the flow and overall recovery of gas is paramount to ensure efficient exploitation. In this work, a comprehensive review of the recent strategies used in analyzing the hydraulic fracturing, storage, flow, and recovery of gas is presented. To begin with, the experimental, analytical, and numerical approaches pertinent to hydraulic fracturing are deeply explored. Additionally, the flow of gas through the newly opened channels is accounted for by using a quadruple-domain approach where the mechanisms of flow in shale reservoirs at the nanoscale, microscale, mesoscale, and macroscale are considered. Furthermore, a strategy to capture the multiple phases, including gas, oil, and water, and recover both carbon dioxide and methane is explored through thermal and enhanced gas recovery approaches. This review provides a baseline for understanding how the hydraulic fracture evolves and propagates to create new channels that contribute to the flow of gas, how the gas flows through the created channels across the many scales of the shale reservoir, and how to improve recovery from shale reservoirs.

Section: Storage and Transport Mechanism Of Gas In Shale Reservoirsmentioning

confidence: 99%

Section: Storage and Transport Mechanism Of Gas In Shale Reservoirsmentioning

confidence: 99%

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Hydraulic Fracturing and Enhanced Recovery in Shale Reservoirs: Theoretical Analysis to Engineering Applications

et al. 2023

“…Shale oil/gas reservoir usually has low-porosity, low-permeability, and complex pore structure, which belongs to typical unconventional reservoir, and hydraulic fracturing is regarded as a necessary way to effectively exploit its resource. − During hydraulic fracturing, the flowback fluid has a higher salt ion concentration in some shale reservoirs, which is totally different from conventional reservoirs. − This special characteristic can reflect the reservoir information, such as the fracture network complexity. , …”

Section: Introductionmentioning

confidence: 99%

Insights into Salt Ion Diffusion Behavior in Imbibed Fluids and the Source of Salt Ions in Clay-Rich Shale from Upper Cretaceous Qingshankou Formation, Songliao Basin

Meng

Shi

et al. 2023

Shale oil/gas is a type of unconventional energy and an important supplement to the energy structure worldwide. Hydraulic fracturing plays a necessary role in exploiting this kind of resource, and a higher salt ion concentration exists in flowback fluid, which is totally different from conventional reservoirs. To understand the special mechanism of high salt ion concentration, the effect of testing conditions on salt ion diffusion behavior by a Mettler SevenExcellence multiparameter conductivity instrument and then salt ions in soaked solution are analyzed. The study samples are selected from lacustrine shale from the Upper Cretaceous Qingshankou Formation. The testing conditions include measuring sites, stirring or not, particle size, sample weight, solution volume, and initial solution concentration. The experiment has perfect repetition as long as 238 days. The conductivity measured at the upside is lower than the one measured at the downside before 10 days and becomes similar after 10 days. Stirring before measurement increases the salt ion diffusion rate obviously before 10 days, which has less influence on the late period. The smaller the particle size is, the higher the conductivity is. The ratio between conductivity and sample weight decreases with sample weight increase. The higher the liquid volume is, the higher the total salt ion in liquid is at last. The initial salt ion concentration has less influence on the salt ion diffusion behavior, which is mainly controlled by its own physical property. Salt ion diffusion needs a long time to approach stability, and the conductivity in some samples is still increasing after 218 days. The main anions in soaked solution are SO4 2–, Cl–, and NO3 –, and the main cations are Na+, Ca2+, Mg2+, K+, Si4+, and Sr2+. Cl– and Sr2+ are mainly from precipitated salt ions in pores, and other salt ions are partially from precipitated salt ions in pores. Our study contributes to understanding the influencing factors for evaluating the salt ion diffusion behavior in the shale reservoir and clarifying the salt ion component in soaked solution, which is conducive to the explanation of flowback fluid with a higher salt ion concentration in clay-rich shale reservoirs. Our research contributes to the selection of high productive shale oil formation.

“…Marine shale gas has been widely developed in the United States and the Fuling gas field of southern China. , Hydraulic fracturing is a central technique to exploit natural gas from shale reservoirs. − From the field experience, a shale well with high production usually has a higher fracturing fluid retention, which could not be displaced out of formation under conventional flowback measures. − Therefore, understanding the water occurrence is beneficial to understanding the retention mechanism. At present, nuclear magnetic resonance (NMR) was widely used to appraise water occurrence, and the T 1 – T 2 map can qualitatively differentiate fluid states, including movable fluid, adsorbed fluid, crystal fluid, and so on. − At the same time, the T 2 spectrum associated with centrifugation can quantitatively distinguish movable water and irreducible water, which has been widely used to appraise water movability. , Pore structure characterization is a basic aspect of understanding reservoir property.…”

Section: Introductionmentioning

confidence: 99%

Insight into Water Occurrence and Pore Size Distribution by Nuclear Magnetic Resonance in Marine Shale Reservoirs, Southern China

Meng

Shen

et al. 2022

The exploitation of shale gas has been a hot topic, and the understanding of water occurrence and pore size distribution in shale reservoirs plays a crucial role in efficiently developing this type of resource. In this study, we target moderate-shallow marine shales from the Lower Cambrian Niutitang Formation and deep marine shales from the Lower Silurian Longmaxi Formation. Low-field nuclear magnetic resonance is applied to investigate the water occurrence and pore size distribution together with complementary experiments including mineral component analysis, scanning electron microscopy (SEM), contact angle measurement, and high-speed centrifugation. The SEM analysis shows that the moderate-shallow shales have some inorganic nanopores and cracks but almost no organic pores, while the deep shales have abundant organic nanopores. The average movable water saturation of moderate-shallow and deep shales is only 15.5% and 15.2%, respectively. It implies that the high irreducible water saturation is a typical characteristic for both marine shales, which is ascribed to the strong capillary force of the nanopore inhibiting water flowing out of the pore space. By means of high-speed centrifugation and contact angle measurement, the converting factor of the T 2 value into pore size was obtained. The average pore size in moderate-shallow shales is 19.4, 21.8, and 26.8 nm, respectively, while the average pore size in deep shales is 109, 57, and 68.4 nm, respectively, for the tested samples. This indicates that organic pores in deep shales contribute to the development of abundant nanopores. This study brings insight into characterizing the water occurrence and pore size distribution of moderate-shallow and deep marine shales.