The Influence of Acid on the RockMechanical Characteristics of Deep Shale in theWujiaping Formation

The microscopic characteristics and mechanical properties of rocks change after the action of acid on deep shale,which affects the fracturing effect.Accordingly,we designed and conducted indoor experiments related to the changes in macro and microscopic characteristics after the interaction of acid with the shale of Wujiaping Formation,based on which the characteristic law of fracture volume modification after acid fracturing was studied using numerical simulation.The results demonstrate that the pores and fractures are enlarged and the structure is significantly loosened after the acid immersion.And a 15%concentration of hydrochloric acid can effectively dissolve shale.Furthermore,the degree of acid-etching reaction is highly variable because of the different carbonate content,which reveals the strong inhomogeneity of the shale system in the Wujiaping Group reservoir section.After the acid interacted with the shale rock samples,the triaxial compressive strength,elastic modulus,and Poisson’s ratio of shale decreased.Moreover,the evaluation of the effect after acid fracturing simulated by fracturing software revealed that the smaller the value of elastic modulus in shale-based reservoirs,the more favorable the fracture volume modification.This discovery not only provides a theoretical basis for the expansion and extension patterns of acid-fracturing in carbonaceous shale formations but also offers research methods and theoretical insights for the fundamental exploration of other deep-seated oil and gas resources.

Formation of fractures in carbonate rocks by pad acid fracturing with different states of carbon dioxide

Carbonate outcrops were taken from Ma 51 sub-member in the Lower Paleozoic in the Yan’an gas field to conduct true tri-axial hydraulic fracturing experiments with water, liquid CO_(2) and supercritical CO_(2). CT scan was applied to analyze initiation and propagation laws of hydraulic fractures in carbonate rocks. The experiments show that supercritical CO_(2) has low viscosity, strong diffusivity and large filtration during fracturing, which is more liable to increase pore pressure of rocks around wellbore and decrease breakdown pressure of carbonate rocks. However, it would cost much more volume of supercritical CO_(2) than water to fracture rocks since the former increases the wellbore pressure more slowly during fracturing. For carbonate rocks with few natural fractures, tensional fractures are generated by fracturing with water and liquid CO_(2), and these fractures propagate along the maximum horizontal principal stress direction;while fracturing with supercritical CO_(2) can form shear fractures, whose morphology is rarely influenced by horizontal stress difference. Besides, the angle between propagation direction of these shear fractures near the wellbore and the maximum horizontal principal stress is 45°, and the fractures would gradually turn to propagate along the maximum horizontal principal stress when they extend to a certain distance from the wellbore, leading to an increase of fracture tortuosity compared with the former. For carbonate rocks with well-developed natural fractures, fracturing with fresh water is conducive to connect natural fractures with low approaching angle and form stepped fractures with simple morphology. The key to forming complex fractures after fracturing carbonate rocks is to connect the natural fractures with high approaching angle. It is easier for liquid CO_(2) with low viscosity to realize such connection. Multi-directional fractures with relatively complex morphology would be formed after fracturing with liquid CO_(2).

Application of multipole array sonic logging to acid hydralic fracturing

Multipole array sonic logging tools have widely been employed in Chinese oilfields in recent years. We have developed a software package for rock mechanical analysis with multipole array sonic logs. This advanced data processing method and software have been applied to the Tahe oilfield in Northern West China to provide guidance to acid hydraulic fracturing design and evaluation. In this paper, we present the field examples of such data processing and applications to demonstrate the validity and advantages of our method and software package.

Diverting agents in the oil and gas industry:A comprehensive analysis of their origins,types,and applications

During the life of a well,treatments are carried out to boost productivity by stimulating initially unproduced zones.These treatments include hydraulic fracturing,matrix acidization,and acid fracturing,among others.Hydraulic fracturing treatment is generally applied to deeper reservoirs of oil or natural gas for enhanced recovery.By infusing proppant,water,and chemicals under extreme pressure during the fracturing procedure,fissures in and beneath the reservoir layer can be accessed and expanded.Another stimulating procedure,matrix acidization,involves injecting acid down the drilling hole to permeate the rock fissures at stresses lower than the fracture stress.In addition,carbonate reservoir acid fracturing stimulation is commonly used as an acid treatment technique whereby a pressure greater than the formation disintegration pressure or spontaneous fracture closure pressure is used to compress acid into the reservoir.These treatments allow existing wells to sustain hydrocarbon production without new wells being drilled.Diverters,when employed efficiently,can prevent the need to use a rig to provide momentary physical barriers,thus lowering the cost of the workover.Recent improvements in diversion technology make use of a variety of degradable particles that act as momentary bridges,either at the perforation entries or inside the existing fractures.The aim of this study is to introduce different types of mechanical and chemical diverters used to enhance the productivity of wells.This study explains the concepts of different types of diverters and their applications in several formations,it will also help readers to understand the selection procedures based on the suitability and requirements of diverter use by case studies from around the world.

Wormholes propagation for fractured-vuggy formation:Laboratory tests,numerical simulation and field application

The propagation of wormhole is vital important for matrix acidizing and acid fracturing in carbonate reservoirs.While the formation of acid dissolved wormhole is derived from heterogeneous physical and chemical transportations and reactions.Alveolate dissolved pores,krast caves,and natural fissures are the major reservoir spaces for the Sinian dolomite formation in the Anyue gas field of the Sichuan Basin.There were four categories of formation,which are matrix dominated,inter-breccia dissolved pore dominated,dissolved pore and cave dominated,and fissure and cave dominated,based on the development intensity and connectedness of caves and fissures.The caves and fissures make the wormhole formation and propagation particularly complicated.Firstly,the 3-D topological structure of dissolved pores,vugs,fissures and throats inside cores is quantitatively scanned by CT imaging technology for its feature of vivid and damage-free.Secondly,3-D patterns of wormhole are obtained with CT scanning after core flooding by acid.Additionally,the porethroat network model is reconstructed with digital cores technology.Then,the size and ratio of pore and throat before and after core flooding by acid is analyzed and the absolute permeability of pore scale flow is numerically simulated to understand the fundamental influence of pores and vugs distribution and connectedness on wormhole propagation.Lastly,the wormhole pattern gained by CT scanning and simulating with two-scale model is compared.Meanwhile,the corrected two-scale model is utilized to simulate the wormhole propagation for matrix acidizing and acid fracturing of Sinian fractured-vuggy dolomite in Anyue gas field,Sichuan Basin.The optimized injection rate and volume were in agreement with the characteristic matrix acidizing operating curve,which indicates that the two-scale model was suitable for matrix acidizing optimization design of such formations.In addition,the simulated acid etched fracture length with considering the dynamic wormhole leakoff was consistent with the well testing interpreted result.

Pressure transient analysis for a fractured well in a stress-sensitive tight multi-medium oil reservoir

Tight multi-medium oil reservoirs are the main source of hydrocarbon resources around the world.Acid fracturing is the most effective technology to improve productivity in such reservoirs.As carbonates are primarily composed of dolomite and calcite,which are easily dissolved by hydrochloric acid,high-permeability region will be formed near the well along with the main artificial fracture when acid fracturing is implemented in tight multi-medium oil reservoirs.In this study,a comprehensive composite linear flow model was developed to simulate the transient pressure behavior of an acid fracturing vertical well in a naturally fractured vuggy carbonate reservoir.By utilizing Pedrosa's substitution,perturbation,Laplace transformation and Stehfest numerical inversion technol-ogy,the pressure behavior results were obtained in real time domain.Furthermore,the result of this model was validated by comparing with those of previous literature.Additionally,the influences of some prevailing parameters on the type curves were analyzed.Moreover,the proposed model was applied to an acid fracturing well to evaluate the effectiveness of acid fracturing measures,to demonstrate the practicability of the proposed model.

A novel stimulation strategy for developing tight fractured gas reservoir

Conventional stimulation methods such as matrix acidizing,acid fracturing,or proppant fracturing have resulted in products that perform poorly and/or fail within months.Other options,such as water fracs with light sand,give better results but are prohibitively expensive.Mineral composition,brittleness index,stress regime,and petrophysical properties,which are favorable for creating complex fracture networks,can be obtained by geochemical and geomechanical analysis.The extended Reshaw and Pollard criterion shows that hydraulic fractures tend to be arrested by pre-existing natural fractures,and complex fracture networks would be created during fracturing.Additionally,the critical stressed faults theory indicates that the pre-existing natural fractures tend to slip with the shear mode as the pore fluid pressure increases.Rotating disk experiments and conductivity tests with artificial sheared plates have shown that flow channels can be etched at the location of scratches on fracture surfaces.Meanwhile,the carbonate cement in natural fractures can be chelated to form wormhole likely flow channels.Complex fracture networks with sufficient acid etched conductivity can be generated by water fracs with acid.A novel and economical volume stimulation strategy known as network acid fracturing has provided Tarim Oil Company the means to develope ultra-deep,ultra-high pressure,high temperature,and ultralow permeability but fractured gas reservoirs.Post-stimulation production performances of numerous wells with network acid fracturing are comparable to those with stimulated reservoir volumes.

Corrosion damage behaviors of rubber O-rings under simulated acid fracturing conditions

To explore the damage behavior of O-ring in acid environment,a high-temperature and high-pressure(HTHP)autoclave was used to simulate the service environment of O-ring,and then 168h corrosion test of hydrogenated nitrile butadiene rubber(HNBR)and fluororubber(FM)O-rings were carried out.The corrosion damage behaviors of two kinds of rubber O-rings in the acidizing fluid were studied through determining their tensile strength,elongation at break,hardness,permanent compressive deformation,tensile fracture morphology and sealing property.The results showed that the crosssectional area and the compression permanent deformation increased,the tensile strength and hardness decreased when the HNBR and FM O-rings under the free state were subjected to acid corrosion.The elongation at break of HNBR decreased,and that of FM rubber increased greatly.Similar with free state,the HNBR and FM O-rings under sealed state also presented the same variation trend.The decrease in the reliability of the O-rings under the sealed state was less significant than that in the free state.In the test,tensile fractures were mostly brittle fractures,HNBR and FM O-rings had obvious corrosion damages such as deformation and swelling.The results could provide a technical basis for the selection of sealing materials,tool optimization design,and construction work in oil and gas fields.

Optimization of acid fracturing for a tight carbonate reservoir

In this study,acid fracturing treatments were simulated for a tight limestone reservoir within a shale formation using FRACPRO software.The purpose was to investigate the optimum acid fracturing design that leads to a higher fracture etched length and width,and higher fracture conductivity.Moreover,the impact of the rock-acid contact time and whether to consider a post-flush or fluids flowback,on the acid fracturing outcomes were also investigated.A simple geological model was constructed which consists of different lithological layers.Different acid fracturing design scenarios were considered starting from a single stage of acid injection to multi-stage treatment.In multi-stage acid treatment,alternate acid-slickwater injection was considered.Plain HCl acids with different concentrations and other acids that are already included in FRACPRO database were used.The results showed that the acid loss during post-flush is among the main problems of the acid fracturing in tight carbonate shale reservoir.For the single stage of acid injection,it is recommended to flow back the acid after well shut-in instead of considering a post-flush stage.The multi-stage alternate acid slickwater injection reduces or even eliminated the acid loss.However,it is recommended to inject a slickwater before well shut-in to reduce the rock-acid contact time,thus reducing the formation damage.The results also showed that the created fracture etched width decreases and the fracture etched length increases as the fracturing stages increase.In this study,because of the low carbonate layer permeability and compressive strength,a two-stage alternate 28%HCl and slickwater injection with a post-flush stage is recommended.

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.

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.