Experimental Study On Full - Diameter Core Slurry Pollution In T - Gas Field

Entrance of solid phase and liquid phase into reservoir during drilling may cause blockage of effective pores and several sensibility damage and reduce the permeability, destruct reservoir structure, lower formation fluid mobility, which can result in reduction of recovery and be unfavorable for development of oil and gas resources as well as influence the economic effect of development and exploitation. T is a typical gas field with low porosity, low permeability, fractures developing in some regions and deep tight sandstone. Before production, some wells’shut-in pressure recovery figures result in a large pollution skin factor and serious pollution characteristics, showing that we are lack of recognition of mud pollution mechanism while drilling. Conventional fluid pollution evaluation method is carried on small size cores (D=2.54cm) under low temperature and pressure, which determines restriction on measurement accuracy and representativeness on reservoir with fractures. We study this project utilized with the methodology on combination of macroscopic and microscopic.Firstly, a mud pollution assessment experiment apparatus of full diameter cores has been established. We use new drilling mud dynamic filtration apparatus and modify the conventional probe of the core holder.Secondly, the mud pollution assessment method has been improved. The cores are totally gas saturated after establishing irreducible water saturation. During the experiment, nitrogen has been used to increase confining pressure instead of hydraulic oil in order to avoid double pollution.Thirdly, we fill the fractures with multilayer screens in order to relieve the core compaction which usually causes reduction of fracture conductivity.Fourthly, a full-diameter core fluid pollution experiment is conducted in this study, which covers comparison of different kinds of reservoir (porous reservoir&fracture-pore reservoir), mud types (oil-based&water-based) under different pressure difference as well as crack width, to estimate formation damage. Research shows that:compared with porous reservoir, fracture-pore reservoir mud filtration and invasion of solid phase particle size is bigger, and formation damage is more serious; the pressure difference is the critical effect to determine damage degree and bigger pressure difference leads to higher permeability recovery degree, higher damage ratio and more serious damage situation; fracture width is a decisive effect to determine the damage degree for fracture-pore reservoir and the wider the fracture is, the lower permeability recovery rate will be, causing higher damage ratio and more serious damage.Finally, we choose three typical cores to study the damage mechanisms, dynamic procedure and influencing factor about drilling mud invasion, including particles and filtrate, by means of scanning electron microscope(SEM) and energy dispersive spectrometry (EDS). The results indicate that, firstly, mud damage concentrate in end face of the cores, with a depth of3centimeters; secondly, the fracture face damages more seriously while the bed rock slightly; finally, the main factor for foamation damage, in T gas field, would be particles rather than water sensitive mineral.The research results have great significance to optimize drilling pressure differential and mud type in latter operation of the T gas field.