Water phase trapping is the most vital damage to tight sandstone reservoirs, which is different from water blocking, and it is a key barrier to improve resource assessment, successful exploration and efficient development operations. It cannot be evaluated by the traditional method evaluating water blocking. This paper takes example from Daniudi tight gas reservoir in northern Ordos Basin, China. The potential tendency to water phase trapping damage is intense by the research on the geology characters such as abnormal low water saturation, pore structure. The severity of water trapping in Daniudi gas filed was proved by the experiments of the effects of water saturation on gas relative permeability, spontaneous imbibition and phase retention of water and dill-in fluids. Based on the influence mechanisms of initial water saturation, reservoir pressure, liquid properties and pore structure on water phase trapping, dimensionless phase trapping coefficient (PTC) was put forward to predict water phase trapping damage potential. The saturation and invasion depth of filtrate varying with time around wellbore were simulated, the results show that when there is no filtrate cake on the well wall the Daniudi gas reservoirs, especially P1x2+3 gas reservoir is prone to be damaged by water trapping to be wrongly evaluated or to be undervalued. In view of the theories and research results, serial measurements to prevent and treat water phase trapping are adopted to protect gas reservoirs and P1x2+3 gas reservoirs was detected to become the major pay formation. The practices of gas formation damage control verified the results of evaluation.
Introduction
The large scale development of tight gas is of profound significance, but the unfavorable geologic status and prone damage greatly limit its development Water phase trapping is defined as the permeability reduction process of near wellbore reservoirs and fracture faces when water saturation is from initial water saturation to irreducible water saturation and to 100% during the process of well operation (Bennion, 1996). Water phase trapping induced by drill-in fluids filtration invasion can change the initial reservoir conditions, which hinders evaluation and economic development of reserves (Kang, 2000). Water phase trapping damage could induce fluid sensitivity damage and aggravate stress sensitivity damage and other damages, which bring tight gas reservoirs one disaster after another (You,2006). Water block neglects the fact that the initial water saturation of gas reservoir is lower than the irreducible water saturation in tight gas reservoirs. So water phase trapping is difference from water block, and the evaluation of water phase trapping damage cannot use the permeability damage ratio which is often used to evaluate liquid sensitivity and water block.
Bennion (1996) provided the phase trapping index (APTi), whose basic formulation is based solely upon permeability and initial water saturation values. This basic formulation is widespread use, but the reservoir pressure, liquid property is not taken into consideration in this basic formulation.
The traditional methods such as permeability damage ratio and aqueous phase trapping index (APTi) have their shortages to precisely predict water phase trapping, especially that of low permeable or tight reservoirs. The measurement of permeability damage ratio of tight reservoirs rocks is time consuming, and the measurement error is large. The measurement of permeability damage ratio does not also take consider of the effect of permeability, initial water saturation, reservoir pressure and rock wettabilty on aqueous trapping.
The phase trapping is a complex process, and is the main damage mechanism to oil and gas reservoirs, especial tight gas reservoirs which have abundant resource and huge development potential and are also a main development objective in these years, so a or serious new evaluation methods is urgent.
