Influence of Gas and Oil State on Oil Mobility and Sweet Spot Distribution in Tight Oil Reservoirs From the Perspective of Capillary Force

Abstract

The coexisting free gas, solution gas and oil in a tight oil reservoir, constrained by the capillary force of the micro-nano pore-throat system, is of crucial importance to the sweet spot of the tight oil reservoir. Equations of modified total capillary force for the liquid phase are proposed to indicate the tight oil mobility in situ, which are derived as follows. (1) New variables representing the free gas volume ratio in pore space, ‘y_i-free’, and the solution gas volume ratio in the liquid phase, ‘y_i-solution’, are proposed to modify the vapor pressure in the Kelvin capillary force equation, based on the Peng-Robinson and Van-Laar equations. (2) Relationships between the vapor-liquid state, phase flow state for production and gas-to-oil production ratio (GOR) are established based on the modified capillary force function curve, in correspondence with the production zones (i.e., the oil zone, wet gas zone and dry gas zone), according to the North American and Yanchang tight production data. The following conclusions can be drawn. (1) We proposed a modified total capillary force function to indicate the tight oil mobility in-situ, based on the EOS (Peng-Robinson Equation) and EOS+λ(Van-Laar Equation) methods, composed of the variables of free gas volume ratio in the reservoir, solution gas volume ratio in the liquid phase, and other geological parameters such as the formation pressure, temperature, wettability coefficient, rock particle diameter and pore diameter. (2) A relationship between the GOR, the gas & oil state and phase flow state for production was established, based on the modified capillary force function and the production data of Yanchang and North American tight oil fields. Five phase stages are identified according to the gas volume ratio (y_i) in situ, e.g., a pure liquid phase in a single oil flow for production, a mixed gas & oil state in a multiphase transient flow for production, a mixed gas & oil state in multiphase stable flow for production, a condensate gas state in a multiphase super-critical flow for production, and a dry gas state in a single gas flow for production. (3) Tight oils are more mobile in three stages according to the capillary function curve in situ: around the critical point of y_{i-critical(1)}, which corresponds to a single oil flow for production and a tight oil sweet spot for the oil zone, such as Yanchang and Bakken tight oils; at the transition zone between oil and wet gas zones before y_{i-critical(2)}, which is in correspondence with a multiphase stable flow state and a sweet spot for the wet gas zone, such as the Eagle Ford tight oil; and between the y_{i-critical(2)} and y_{i-critical(3)} in a condensate gas state, which is in correspondence with a multiphase super-critical flow for production and a sweet spot at the transition zone between wet gas and dry gas zones.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019