Fracturing-flooding technology for low permeability reservoirs:A review

The development of low-permeability oil and gas resources presents a significant challenge to traditional development methods.To address the problem of“no injection and no production”in low-permeability reservoirs,a novel fracture-injection-production integration technology named fracturing-flooding has been proposed by oilfield sites.This technology combines the advantages of conventional fracturing,water flooding,and chemical flooding,resulting in improved reservoir physical properties,increased injection,replenished energy,and increased oil displacement efficiency.The technology is especially suitable for low-permeability reservoirs that suffer from lack of energy,and strong heterogeneity.Fracturing-flooding technology has shown significant results and broad development prospects in some oilfields in China.This paper analyzes the development status of fracturing-flooding technology from its development history,technical mechanism,technical characteristics,process flow,types of fracturing and oil displacement fluids,and field applications.Physical and numerical simulations of fracturingflooding technology are also summarized.The results suggest that fracturing-flooding technology is more effective than conventional fracturing,water flooding,and chemical flooding in stimulating lowpermeability tight reservoirs and improving oil recovery.Moreover,it has a high input-output ratio and can be utilized for future reservoir stimulation and transformation.

Effect of proppant sizes and injection modes on proppant transportation and distribution in the tortuous fracture model

Particle-fluid transport and placement mechanism in tortuous fracture played a crucial role in uncon-ventional reservoirs.Currently,most studies focused on mono-size proppant with fluid transport pro-cesses in tortuous fractures.However,the mixture-size proppant with fluid movement mechanism in tortuous fracture was still uncommon.Therefore,this study designed and applied a series of experiments with a physical analog model of a tortuous fracture with 120°and 90°-angled bends and combined high-speed camera-based equipment.This experimental system was used to track different-mixture-sized proppant particle motion trajectories for a series of proppant injection schemes;The following conclu-sions were drawn from this study:1.The pile-up processes mechanism in all investigated schemes were similar and could be reduced to four main stages.2.The packing structure at both sides of the fracture wall had different variation rates,which were controlled by the mix ratio(change from 1∶1-1∶5)of proppant size.3.Some new packing patterns,such as Zebra Stripe,had occurred,controlled by the different proppant injection sequences.4.Small-sized mono-proppant(30/50 mesh)had the highest transport efficiency in the tortuous fracture,followed by the mixed-sized multi-proppant(10/20 mesh:30/50 mesh),large-sized proppant(10/20 mesh)was the worst.5.An optimized alternating in-jection mode was recommended as injecting small-sized proppant first(30/50 mesh)and followed by mixed-sized multi-proppant(10/20 mesh:30/50 mesh),which could contribute to obtaining the optimal both proppant packing height and travel distance in tortuous fracture.6.Two correlations were devel-oped for predicting the proppant packing height and transportation distance.

Feasibility study on application of volume acid fracturing technology to tight gas carbonate reservoir development

How to effectively develop tight-gas carbonate reservoir and achieve high recovery is always a problem for the oil and gas industry.To solve this problem,domestic petroleum engineers use the combination of the successful experiences of North American shale gas pools development by stimulated reservoir volume(SRV)fracturing with the research achievements of Chinese tight gas development by acid fracturing to propose volume acid fracturing technology for fractured tightgas carbonate reservoir,which has achieved a good stimulation effect in the pilot tests.To determine what reservoir conditions are suitable to carry out volume acid fracturing,this paper firstly introduces volume acid fracturing technology by giving the stimulation mechanism and technical ideas,and initially analyzes the feasibility by the comparison of reservoir characteristics of shale gas with tight-gas carbonate.Then,this paper analyzes the validity and limitation of the volume acid fracturing technology via the analyses of control conditions for volume acid fracturing in reservoir fracturing performance,natural fracture,horizontal principal stress difference,orientation of insitu stress and natural fracture,and gives the solution for the limitation.The study results show that the volume acid fracturing process can be used to greatly improve the flow environment of tight-gas carbonate reservoir and increase production;the incremental or stimulation response is closely related with reservoir fracturing performance,the degree of development of natural fracture,the small intersection angle between hydraulic fracture and natural fracture,the large horizontal principal stress difference is easy to form a narrow fracture zone,and it is disadvantageous to create fracture network,but the degradable fiber diversion technology may largely weaken the disadvantage.The practices indicate that the application of volume acid fracturing process to the tight-gas carbonate reservoir development is feasible in the Ordovician Majiagou Formation of lower Paleozoic,which is of great significance and practical value for domestic tight-gas carbonate reservoir development and studies in the future.

Productivity model for gas reservoirs with open-hole multi-fracturing horizontal wells and optimization of hydraulic fracture parameters

Multi-fractured horizontal wells are commonly employed to improve the productivity of low and ultra-low permeability gas reservoirs.However,conventional productivity models for open-hole multi-fractured horizontal wells do not consider the interferences between hydraulic fractures and the open-hole segments,resulting in significant errors in calculation results.In this article,a novel productivity prediction model for gas reservoirs with open-hole multi-fractured horizontal wells was proposed based on complex potential theories,potential superimposition,and numerical analysis.Herein,an open-hole segment between two adjacent fractures was regarded as an equivalent fracture,which was discretized as in cases of artificial fractures.The proposed model was then applied to investigate the effects of various parameters,such as the angle between the fracture and horizontal shaft,fracture quantity,fracture length,diversion capacity of fractures,horizontal well length,and inter-fracture distance,on the productivity of low permeability gas reservoirs with multi-fractured horizontal wells.Simulation results revealed that the quantity,length,and distribution of fractures had significant effects on the productivity of low permeability gas reservoirs while the effects of the diversion capacity of fractures and the angle between the fracture and horizontal shaft were negligible.Additionally,a U-shaped distribution of fracture lengths was preferential as the quantity of fractures at shaft ends was twice that in the middle area.

General criterion for intersection between hydraulic induced fractures and pre-existing natural fractures

Pre-existing natural fractures and other structurally weak planes are usually well-developed in unconventional reservoirs.When such fractures intersect with hydraulic induced fractures,they will redirect and propagate as an important mechanical principle of volume fracturing by the formation of complex fracture networks.Under the shadow effect of natural fractures and other structurally weak planes with hydraulic supported fracture stress,hydraulic fractures do not fully propagate in the direction of the maximum horizontal-principal-stress.This paper computed the stress intensity factors of hydraulic fracture types I and II by integrating the various interactions,established universally-applicable mechanical principles for the propagation behavior when a hydraulic fracture propagating in an arbitrary direction intersects with a natural fracture at an arbitrary angle,and demonstrated the mechanical principles of the intersection between hydraulic induced fractures and pre-existing natural fractures.This study proved the following conclusions:as the intersection angle between the hydraulic fracture and the maximum horizontal-principal-stress increased,the possibility of the hydraulic fracture being captured by the natural fracture with an identical approaching angle first increased and then decreased;as the net stress increased,the intersection behavior between the hydraulic fracture and the natural fracture transitioned from penetration to capture.

Investigation of the degradation and stability of acrylamide-based polymers in acid solution: Functional monomer modified polyacrylamide

Acrylamide copolymers are often used as acidizing diverting and thickening agents for their advantageous thickening,flocculation,adhesion and resistance reduction properties.Experimental results indicate that the acid concentration greatly affects the properties of acrylamide polymers,which varies from results reported by other researchers.Considering the theoretical and field application value of the present study,four comparable acrylamide-based polymers were synthesized,and their macro-and micro-changes as well as the related changes in viscosity and molecular weight were studied in high-concentration hydrochloric acid.A proposed mechanism of acrylamide copolymer stability and degradation is provided,and further suggestions are made for the modification of acrylamide copolymers.

Experimental study on influencing factors of acid-fracturing effect for carbonate reservoirs

Acid fracturing treatment is the key technique for stimulation and stable production in carbonate reservoirs.In order to improve the carbonate reservoirs acid fracturing effect,in this paper,with a large number of experiments as the main research methods,study on influencing factors of acidfracturing effect for carbonate reservoirs from increase the effective distance of living acid,increase acid corrosion eched fracture conductivity,reduce the acid fluid loss,etc.The effective distances of live acid calculated with reacted acid limitations measured in different acid systems are quite different from those calculated according to previous standard.Fracture conductivity is one of the key parameters that affects acid fracturing effects,but it's difficult to be predicted accurately due to the strong randomness of acid-rock reaction as well as various influence factors.Analyses of the impacts on fracture conductivity resulted from the rock embedment intensity,closure stress,acid dosage,rock-acid contact time,acid fluid loss,acid pumping rate through self-developed small-core fracture capacity test instrument.Fluid loss during acid fracture can be well controlled by thickened liquid as well as solid particles,but formation damage occurs inevitably.Foamed acid is a specific fluid with high viscosity,low fluid loss,small friction resistance,good retarding property,strong fracture making ability,easy flowback and low damage,which is an ideal acid system for low pressure and low permeability carbonate reservoirs.In this paper,the theoretical study on percolation mechanism and fluid-loss control mechanism of foam(acid)in porous medium are presented with the help of visual microscopic model fluid drive unit.

Technical status and challenges of shale gas development in Sichuan Basin,China

During the past decade,shale gas developments have changed the energy structure in the US natural gas industry,and the exploration activities for shale gas are also increasing worldwide.According to the papers published in recent years,shale gas resources are quite abundant in China.With the successful experience obtained from North America,many state-of-the-art technologies are brought in and refined for field application.State-owned enterprise,private enterprises and foreign enterprises have all actively participated in the exploitation of shale gas.Compared with US,China faces many more challenges,both geological and above-ground,in the development of shale gas resources.This paper begins with the introduction of shale gas reserve distribution in China and the identified shale gas formation in Sichuan Basin.The following paper reviews the methodology employed in the geophysical prospecting,drilling and completion,and hydraulic fracturing process.Since China is in the early stage of shale gas development,there is a great technical gap between China and North America.Based on literature review,the major challenges faced in the exploration and production process are identified.What presented in this paper should be of particular interest to the personnels involved in shale gas production in China and countries that are about to set foot in shale gas business.It will also be of interest to researchers who are dedicated to developing these technologies to unlock unconventional gas resources in China.

Numerical simulation of foam diversion acidizing in heterogeneous reservoirs

Foam diversion acidizing can effectively solve the problem of acid distribution with severe heterogeneity between and within layers.Based on the foam diversion principle,the gas trap theory,and volume conservation principle,the foam slug diversion acidizing model was established and solved considering the change of bottomhole temperature and deviation factor of foam.The simulation results show that the change of temperature has a great influence on the diversion effect at the initial stage of injection,but a small influence at the middle and late stage.The effect of temperature on the highly permeable layer is greater than that of temperature on the low permeability layer.The deviation factor of foam is mainly controlled by temperature at the initial stage,and by pressure at the middle and late stage,and the whole process shows a downward trend,which has little influence on the diversion effect.The quasi-skin factor of gas trap is the most important parameter that influences the effect of foam diversion.The water saturation of the low permeability layer rises faster than that of the high permeability layer,and the effect of diversion is obvious.The research results have a strong guiding significance for foam diversion acidizing.