Dag Chun Standnes

Wettability alteration in chalk 2. Mechanism for wettability alteration from oil-wet to water-wet using surfactants

Wettability alteration in chalk 2. Mechanism for wettability alteration from oil-wet to water-wet using surfactants

Wettability alteration in chalk. 1. Preparation of core material and oil properties

A reproducible method has been developed to create homogeneous oil-wet chalk material wettability with and without initial water saturation. This was accomplished by aging crude oil-saturated chalk cores for different time intervals and at different temperatures. A total of five different crude oils were tested for their potential to alter the wetting properties of the chalk material. Chemical analysis of the crude oils showed that the potential for wettability alteration correlated with the amount of acidic components in the crude oil, as measured by their acid number (AN). The wettability of the chalk was evaluated by their ability to spontaneously imbibe synthetic brine. There was no sign of water imbibition into cores without initial water saturation after 5 weeks, while cores with an initial water saturation of 23% imbibed about 9% of water during 8 weeks at 40°C.

Wettability alteration in carbonates

Abstract The chemical interaction between cationic surfactants of the type R-N(CH3)3Br, termed CnTAB, and carboxylates is the key factor in changing the wettability from oil-wet chalk to more water-wet conditions. Oil can then be displaced from the chalk by spontaneous imbibition of water. Carboxylates from the crude oil are the most strongly adsorbed material onto the chalk surface, and they may act as “anchor” molecules for other surface-active components present in the crude oil. This paper focuses on the ion-pair interaction between the cationic surfactant and the carboxylates present in crude oil and model oil systems made by dissolving fatty acids (octanoic, lauric and stearic) in heptane and crude oil. Partitioning of the cationic surfactant between the oil and the water phase was studied as a function of the type and amount of acid present, pH, and composition of the brine. By means of static contact angle measurements, it is verified that the concentration of surfactant is very important in desorbing carboxylates from the calcite surface. The process nearly stops at surfactant concentrations below the critical micelle concentration, indicating that the desorbed carboxylates must be stored in micelles or extracted into the oil phase in order to maintain a dynamic wettability alteration process in a porous medium. Dynamic experiments, using model oil systems, containing different types of fatty acids and C12TAB dissolved in brine, showed that the surfactant solution imbibed spontaneously into the oil-wet material in a counter-current flow regime governed by mainly capillary forces, indicating that a wettability alteration process had taken place.

Spontaneous imbibition of water into oil-wet carbonates

Contrary to sandstone reservoirs, the oil-wetting nature of carbonate reservoirs appears to increase as the reservoir temperature decreases. Thus, a carbonate reservoir at Tres<50 °C containing an oil with an acid number (AN) of about 1 mg KOH/g oil is very likely to behave as oil-wet. If, in addition, the reservoir is fractured with low permeability matrix blocks, water flooding of the reservoir is not recommended unless a wettability alteration process is possible. Normally, the oil recovery from this type of reservoirs by natural pressure depletion is low, and therefore the potential for improved oil recovery (IOR) is very high. The present paper reviews some recent work focused on improved oil recovery from oil-wet carbonates using surface-active chemicals to promote wettability alteration. Water will then spontaneously imbibe into the matrix blocks, and the oil recovery can be markedly increased. The following topics are discussed: • Relative affinities of crude oil components towards carbonates, and how to prepare homogeneous oil-wet cores from water-wet outcrop material; • The wettability alteration mechanism using CTAB, emphasizing ion-pair formation as a key factor; • The relative influence of capillary and gravity forces on the fluid flow during the imbibition process; • The efficiency of commercially available technical products at low price suitable for field applications.

Nontoxic low-cost amines as wettability alteration chemicals in carbonates

Abstract It is possible to improve the oil recovery from low temperature, oil-wet/neutral-wet, fractured carbonate reservoirs by spontaneous imbibition of water using nontoxic low-cost primary amines, R–NH2, as wettability alteration chemicals added to the injection water. The present paper discusses possible limitations in the performance of the chemicals such as solubility, pH, carbonate dissolution, and temperature effects. It was observed that C10-amine was compatible with high salinity brine at pH

Improved Modeling of Gravity-Aided Spontaneous Imbibition Using Momentum-Equation-Based Relative Permeabilities

It is well known that relative permeabilities (RPs) can vary depending on the flow configuration and are lower during counter-current flow as compared to co-current flow. In this paper we use a novel two-phase momentum-equation approach to generate effective RPs where this dependence (and others) is well captured whereby the fluids transfer momentum due to fluid-rock interaction and fluid-fluid interaction. During co-current flow the faster moving fluid accelerates the slow fluid, but is itself decelerated, while for counter-current flow they are both decelerated. We investigate recovery of oil from a matrix block surrounded by water due to a combination of gravity drainage (GD) and spontaneous imbibition (SI), relevant for fractured reservoirs. In capillary-dominated systems the flow is counter-current and viscous coupling can result in increased time scale of the recovery process. During gravity-dominated flow it is more co-current and applying co-currently measured relative permeabilities from the lab becomes a better assumption. Using one set of parameters the momentum-equation approach is thus able to model the behavior of blocks of different operating at different Bond numbers in the reservoir.

An Analytical Model for Analysis of Centrifuge Capillary Pressure Experiments

Primary drainage with centrifuge is considered where a core fully saturated with a dense wetting phase is rotated at a given rotational speed and a less dense, non-wetting phase enters. The displacement is hindered by a positive drainage capillary pressure and equilibrium is approached with time. We present general partial differential equations describing the setup and consider a multi-speed drainage sequence from one equilibrium state (at a given rotational speed) to the next. By appropriate simplifications we derive that the process is driven by the distance from equilibrium state as described by the capillary pressure at the inner radius and position of the threshold pressure (transition from two to one-phase) from their equilibrium values. Further, an exponential solution can describe the transient production phase. Using representative input saturation functions and system parameters we solve the general equations using a commercial software (Sendra v2016.1) and compare with the predicted exponential solutions. It is seen that the match is excellent and that variations in time scale are well captured. The rate is slightly underestimated at early times and overestimated at late times, which can be related to changes in total mobility during the cycles for the given input.