Study On Degradation Mechanism And Countermeasures Of Artificial Fracture Conductivity In Carbonate Reservoir Under High Closure Pressure

Tahe oilfield,located at the secondary tectonic unit of Akekule lobe,south of middle Shaya uplift tectonic zone in the north Tarim basin(Wu You,2013),is one of the rare Ordovician fractured vuggy carbonate reservoirs of hadal facies.With the large burial depth(5,400-8,000m),high temperature(120-170 ℃)and high formation fracture pressure gradient(0.18-0.0236Mpa/m),solution fissure constitutes the main reservoir space and the fracture-vug bodies are inhomogeneous with poor connectivity.So it is considerably difficult to conduct oil and gas exploitation and has become a challenge worldwide to carry out exploration and development there.Acid fracturing is a key technique for increasing reserves and production of fractured vuggy carbonate reservoir.75% of wells in Tahe oilfield require acid fracturing modification to connect the fractures and vugs on the zones far from wells to gain productivity.Tuofutai region is situated in the slope belt in the southwest of Akekule lobe.It is discovered,in Ordovician,that there is an oil bearing area of 211.27km~2 and oil geological reserves of 9402.05×10~4t.At present,there are totally 207 producing wells with the average individual well production capacity per day of 31.5t.It is the important guarantee for stable yields in Tahe oilfield.With the deepening of exploration and development in Tahe oilfield,the fracture closure and conductivity,affected by high closure pressure(≥50MPa),degrade;and oil and gas channels have low connectivity.This causes the shortening of flow periods of oil wells(<50d).The wells will soon become those of low production and poor efficiency.Meanwhile,it is of the high cost to conduct sidetracking or drill for fractures and vugs.How to realize the efficient exploitation has become a main problem of oil and gas production stimulation in Tahe oilfield.It is therefore necessary to analyze influencing factors and influencing laws of acid-etched fracture conductivity for acid fracturing wells in this block and further make clear the fracture conductivity degradation mechanism,in order to develop new technology ideas and processing techniques and increase the useful life of acid fracturing to improve individual well production capacity and gain greater economic efficiency and social benefits.Taking as starting point analysis on factors affecting acid-etched fracture conductivity under high closure stress,this paper carried out 99 groups of laboratory experiment to investigate effects of the following 8 aspects on acid-etched fracture conductivity: closure stress,acid concentration,closure time,rock mineral composition,acid-rock contact time,acid fluid type,proppant size and proppant sanding concentration.It confirmed that fluid type,acid-rock contact time,surface shape of fracture and antideformability of fracture were the main influencing factors of acid-etched fracture conductivity.Researching and setting up acid-etched fracture conductivity prediction models suitable for Tuoputai region,it provided theoretical basis for seeking out countermeasures for the degradation of fracture conductivity.Secondly,based on laboratory experiments,it defined that alternating injection of different acid fluids could improve the artificial fracture conductivity.According to the characteristics of large burial depth and high temperature of research area,it researched and synthesized the acid fluid system of deep penetrating retarding crosslinked acid.From the perspective of long-time conductivity decline variation law,it completed 103 relative laboratory experiments.It presented 3 technology ideas to improve the long-term artificial fracture conductivity,including inhomogeneous acid fracturing with alternating acid injection of different acid fluids,high channel sanding recombination acid fracturing and self-support high conductivity acid fracturing.Combining the researched and developed acid system,it evaluated the laboratory experiments and conducted over 130 groups of VOF model numerical simulation calculation.By using the theory of viscous fingering,it simulated the process parameters of alternating injection with different viscosity acids and optimized the non-uniform coefficients.The optimized viscosity ratio between high viscosity and low viscosity fluids was 5.When injecting acid fluids of low viscosity,it was necessary to improve the construction displacement as much as possible.It clarified determining the injection classes and injection displacement of each class according to injection scale of total acid,viscosity difference and construction displacement,improving the fracture conductivity of distant wells.In acid-etched fracture,fiber was used for "packaging" and "restraining" of high intensity proppant slug injection machine.High intensity proppant was injected with the design fiber length of 12 mm,concentration of 0.6%,order of 40/70 and bearing pressure of 86 Mpa.Impulse sanding construction was conducted with the injection displacement of 5m~3/min,slug interval of 2 min and ratio between fiber and proppant of 1:1.“Pillars” were formed inside the artificial fractures to generate the channels with high conductivity;thus,achieving the purpose of improving individual well productivity.It broke through the conventional idea,changing the acid etching pattern of formation rocks.By injecting oil soluble resin not reacting with acid and keeping some discontinuous fracture planes not participating in the acid-rock reaction,it achieved the self-support effect of formation rock,thus forming self-support high conductivity acid fracturing technology.Through experiments,it proved that selfsupport fracture conductivity was higher than normal acid-etched fracture conductivity.When the closure pressure was larger than 60 Mpa,self-support fracture conductivity was higher than normal acid-etched fracture conductivity and proppant fracture conductivity.In accordance to the features of Tuoputai region,concentration of oil soluble resin was recommended to be 20-30%.Finally,it explored and formed the technology series to improve the artificial fracture conductivity that were suitable for ultra-deep super-high temperature carbonate reservoir under high closure pressure.After 6 well times of field implementation,oil production increased by 3.1×10~4t and useful life reached to 167-370 days.With the remarkable implementation effects,it effectively solved the problems of poor acid fracturing in partial reservoir,small room for subsequent development and low exploitation degree of individual well reserve of old processing measures.The new technology series have broad application prospects.