Investigation On Special Seepage Law And Reserves Mobility Of Deep Carbonate Gas Reservoirs

Deep carbonate gas reservoirs have become an important resource for global natural gas supply with the continuous development of fossil fuel exploration and exploitation technologies.Compared with the medium-shallow hydrocarbon reservoirs,the deep carbonate gas reservoirs form a coexistence pattern of multi-type media of pores,fractures and cavities through multi-period of tectonic movement and diagenetic transformation.As a result,this specific pattern leads to deep carbonate gas reservoirs with characteristics of strong heterogeneity,special seepage mechanism,complex gas–water relationship,difficulty in reserves evaluation,and large difference in gas well productivity.The systematic understanding of the special seepage law and reserves mobility of deep carbonate gas reservoirs is prerequisite for the efficient development of such gas reservoirs,and it is also the main study goal of this investigation.In view of these characteristics,a variety of testing techniques and experimental methods have been used to study the pore structure,single-phase flow,gas–water two-phase flow,commingled production and petrophysical property cutoffs,which can provide a certain theoretical basis for the typical engineering problems in the process of efficient development of deep carbonate gas reservoirs.At first,the thin section and the scanning electron microscope were used to qualitatively characterize the lithology,physical property,storage space and throat of the reservoirs.The deep carbonate gas reservoirs are classified into fracture–cavity-type,cavity-type and pore-type according to the characteristics of capillary pressure curves obtained by high pressure mercury intrusion.The two-dimensional and three-dimensional pore structure characteristics of three types of samples were quantitatively characterized based on the multi-scale CT scans.In addition,the storage capacity and seepage capacity of three types of reservoirs are analyzed based on these results.Subsequently,a high-temperature and high-pressure multi-functional experimental platform was constructed to fully simulate the formation conditions of the deep carbonate gas reservoirs in the study area to conduct displacement experiments and depletion experiments.The seepage law of single-phase in the three types of reservoirs was comparatively studied,and the gas mobility in different reservoirs was analyzed.The effect of irreducible water on the single-phase flow of the three types of reservoirs were compared and studied,and the difference in mechanism was also analyzed.Then,three types of micromodels(fracture-,cavity-,and fracture–cavity-type)were primarily designed and constructed using a technique involving a combination of CT scanning and microelectronic photolithography,which could approximately replicate the microstructures of deep carbonate gas reservoirs.The microscopic visual experiments of gas-drive-water and water-drive-gas were conducted to qualitative study gas–water twophase flow mechanism,water encroachment characteristics,water occurrence characteristics,trapped gas and residual water formation mechanism.Image J gray analysis method was utilized to study the distribution of gas–water,saturation and gas recovery intuitively,and the influence of formation water on gas seepage and mobility was also analyzed from the mechanism.Next,a high-temperature and high-pressure commingled production physical simulation experiment was established to fully simulate and study the change rule of single-layer gas production,cumulative production,reservoir pressure recovery efficiency,and productivity contribution.The effects of multilayer heterogeneity,production differential pressure,water saturation,and water encroachment on law of seepage and production were analyzed.Furthermore,the applicability of the experimental results and laws were verified by numerical reservoir simulation.Finally,core physical properties,mercury injection data,gas testing data,laboratory experiment data and other information were collected and integrated.Several improved static methods such as empirical method,cumulative frequency method,porositypermeability relationship method and minimum flow pore-throat radius method were used to analyze and determine the lower limits of porosity and permeability for reservoirs in study area.Then,several dynamic methods,such as production data method,physical property testing method and improved productivity simulation method,were used to verify and amend the petrophysical property cutoffs.A set of relatively complete method for determining the petrophysical property cutoffs of effective reservoirs in deep carbonate gas reservoirs has been established.