Water Saturation in Tight Gas Reservoirs

Abstract

Abstract Assuming a non-equilibrium scenario most of the non-conventional properties of gas-tight reservoirs are fully explained. The model is able to describe, among others, the following typical characteristics of these reservoirs: over-pressurization, "abnormal" low water saturation, unusually small or undetectable capillary transition zones, abnormal pressure gradients, reservoir to reservoir disconnection and absence of identifiable free water level. After accepting de usual over-pressurization as a direct indication of absence of hydrostatic equilibrium, the usual upscaling of capillary pressure curves results meaningless. It is so because capillary pressure was not originated in hydrostatic columns but in over-pressurization occurred when hydrocarbons were expulsed from the source rock (usually in close contact with reservoir rock). As a result, fluids distribution is affected by non-hydrostatic equilibrium conditions still acting at the time of reservoir discovery. A specially designed laboratory routine to measure the water saturation and electric properties directly on cores avoiding the usual water column modeling through capillary pressure curves, is presented. The relative permeability curves validity is also discussed. Introduction Reservoir engineering calculations usually assume equilibrium conditions. Among these, hydrostatic equilibrium is one of the most commonly "accepted" scenarios. In spite of this commonly accepted situation, tight gas reservoirs show several non equilibrium indications. Tight Gas Reservoirs Characteristics. Additional to standard definition based on rock permeability (below 0.1 mD), and economical considerations, the following are some usual technical characteristics of tight gas reservoirs:Significant formation thickness.Isolated reservoirs inside the same formation.Hardly to detect or non existent water transition zones.Independent Free Water Level (FWL) for every reservoir.Over-pressurized reservoirs."Anomalous" water gradients.Water saturation below expected on usual capillary pressure curves interpretation.Co-existence or intercalation of source rock with reservoir rock.… Not all of these characteristics are found in every tight gas reservoirs but different combinations of them are usually present. Among these characteristics a very significant one is the lower than expected water saturation. This situation has received a particular denomination: "Sub-irreducible water saturation"1. Unfortunately, the name "Sub-irreducible" suggests some abnormal process whereas, as it will be shown in this paper, water saturation easily exceeds or meets irreducible condition. It will be shown that measured water saturation is the normal result of the applied capillary pressure while the denomination of "Sub irreducible water saturation" is a misunderstanding based on the assumption of hydrostatic equilibrium inside the reservoir. In fact, the usually found over-pressurization represents a very strong indication that some non-equilibrium situation governs the reservoir. Once detected an over-pressurization condition, it may be assumed that this is a final condition or, as geological events suggest, the reservoir is releasing the excess pressure during "geological times". In the first case, perfect seals, along geological times, must be assumed. The second scenario only assumes that equilibrium has not been reached yet. The main purpose of this paper is to show that non-equilibrium conditions turns irrelevant the concepts of Free Water Level and "Capillary Transition Zone" associated to fluids distribution in hydrocarbons traps. In fact, as it will be shown, both terms become mathematical calculations with no physical support. Capillary Pressure Definitions Different Capillary Pressure definitions exist. Each has its own applicability conditions.