Petrophysical Properties of Unconventional Low-Mobility Reservoirs (Shale Gas and Heavy Oil) by Using Newly Developed Adaptive Testing Approach

Abstract

Pressure testing in very low-mobility reservoirs is challenging with conventional formation-testing methods. The primary difficulty is the over-extended build-up times required to overcome wellbore and formation storage effects. Possible wellbore overbalance or supercharge are additional complicating factors in determining reservoir pressure. This paper addresses the above technical complications and estimates petrophysical properties of low-mobility formations using a newly developed adaptive-testing approach. The adaptive-testing approach employs an automated pulse-testing method for very low-mobility reservoirs and uses short drawdowns and injections followed by short pressure stabilization periods. Measured pressure transients are used in an optimized feedback loop to automatically adjust subsequent drawdown and injection pulses to reach a stabilized pressure as quickly as possible. The automated pulse data is used to determine supercharge effects, formation pressure, and mobility via analytical models by analyzing the entire pressure sequence. A genetic algorithm estimates additional reservoir parameters, such as porosity and viscosity, and confirms results obtained with analytical models (reservoir pressure and permeability). The modeled formation pressure exhibits less than 1% difference with respect to true formation pressure, while the accuracy of other parameters depends on the number of unknown properties. As a quicker method to estimate reservoir properties, a direct neural-network regression of pulse-testing data was also investigated. Synthetic reservoir models for low-mobility formations (M < 1 μD/cp), which included the dynamics of water- and oil- based mud-filtrate invasion that produce wellbore supercharging were developed. These reservoir models simulated the pulse-testing methods, including an automated feedback-optimization algorithm that reduces the testing times in a wide range of downhole conditions. The reservoir models included both simulations of underbalanced and overbalanced drilling conditions and enabled the development of new field-testing strategies based on a priori reservoir knowledge. The synthetic modeling demonstrates the viability of the new pulse-testing method and confirms that difficult properties, such as supercharging, can be estimated more accurately when coupled with the new inversion techniques.

Transient response of near-wellbore supercharging during filter cake growth

“…The supercharging phenomenon in the vicinity of the borehole is significant because it is a prerequisite for analyzing wellbore stability, reservoir damage, and formation testing while drilling; thus, numerous works have been conducted on this topic. It should be noted during drilling that the pressure differential between the wellbore and the formation causes the mud filtrate to invade the formation while depositing solid particles at the wellbore wall to form a filter cake (Banerjee et al, 2006;Ma et al, 2020); a schematic description of the pressure profile with supercharging in the vicinity of the borehole is provided in Figure 1, which shows that the pressure in the wellbore is at hydrostatic pressure (p w ) but falls quickly across the filter cake (p a ) and then slowly decreases in the formation to the original formation pressure (p i ) some distance away from the wellbore (Banerjee et al, 2006;Hadibeik et al, 2012).…”