Professor Ole Torster Wettability in reservoir engineering Knowledge

Professor Ole Torsæter Wettability in reservoir engineering. • • Knowledge for a better world Introduction Factors affecting wettability Core handling Measurement methods

Definitions • When a liquid is brought into contact with a solid surface, the liquid either expand over the whole surface or form small drops on the surface. • In the first case the liquid will wet the solid completely, while in the other case a contact angle >0 will develop between the surface and the drop. Knowledge for a better world

Introduction The wettability of a reservoir rock system will depend on many factors: - Reservoir rock material - Geological mechanisms (accumulation and migration) - Composition and amount of oil and water - Physical conditions; pressure and temperature - Mechanisms occurring during production; i. e. change in saturations, pressure and composition. It is difficult to make a general model of wettability including all these factors. Knowledge for a better world Berea Sandstone Strongly water-wet ɸ: 23% k: 400 m. D

Factors affecting wettability: Rock and water – Rock composition • carbonates are basic • Silicates are acidic • Different minerals have different basic/acidic properties – Water • Presence of water inhibits oil wetting ability • Salinity • Composition • p. H Knowledge for a better world 4

Factors affecting wettability: Oil properties – Oil composition – Resins (NSOs) • Nitrogen • Sulphur • Oxygen – Heavy depositional polar components » asphaltenes, kerogen, bitumen, wax Increase of heavy components = Increase of oil wetness Knowledge for a better world

Factors affecting wettability in the laboratory – Temperature • Initial drop of temperature (and pressure) can alter native wettability – Loss of light ends – more oil wetting? – Deposition of heavy ends, asphaltenes – Redistribution of fluids – Oxidation • Oxidation of crude creates surfactants – Surfactants Knowledge for a better world 6

Core handling and flooding • Accurate core measurements require representative wettability • Core handling can change wettability • Core flooding experiments in the lab are therefore usually performed on socalled restored cores (cleaned and then treated in crude oil for a long time to obtain desired wettability). • Wettability play an important role in the production of oil and gas as it not only determines initial fluid distributions, but also is a main factor in the flow processes in the reservoir rock. Knowledge for a better world 7

Grain Oil Water Wettability nomenclature – Water wet – Oil wet Water wet system Oil-wet system – Fractional wettability (heterogeneous, spotted, dalmatian? ) • Some portions strongly oil wet, rest strongly water wet – Mixed wettability • Large pores oil wet (continuous oil wetting) • small pores water wet – Intermediate wettability • Slight but equal oil or water wetting throughout the core – Neutral wettability • No real preference for either oil or water No spontaneous production Small, equal spontaneous production Knowledge for a better world 8

Measurement of Wettability • No satisfactory method exists for in situ measurement of wettability, and therefore it is necessary to estimate the wettability from laboratory measurements. • To obtain representative information on wetting preferences in the reservoir from laboratory experiments, the following conditions should be fulfilled: • The method should not damage the surface properties of the rock • The method should determine wettability from very water-wet to very oil-wet • The results should not depend on parameters such as rock permeability and fluid viscosity • The results should be reproducible Knowledge for a better world

Measurements on Core Samples • The most common methods for measuring wettability on core samples are: • Displacement test with two different fluids (rel. perm. ) • Capillary pressure measurements (USBM-method) • Measurements of nuclear magnetic relaxation (NMR) rate • Imbibition measurements (Amott) • Imbibition and displacement (Amott - Harvey method). Knowledge for a better world The Amott. Harvey test is the most accepted and widely used test in the oil industry

The Amott-Harvey procedure includes four steps: Displaced oil 1. Spontaneous imbibition: Oil-saturated sample is placed in an imbibition cell surrounded by water. The water is allowed to imbibe into the core sample displacing oil until equilibrium is reached. The volume of water imbibed is equal to the Rock sample oil displaced ; Vo 1 2. Forced imbibition: The core is moved to a core holder and water is pumped through. The volume of oil displaced may be measured; Vo 2 Core holder with rock sample Water Oil Water index: Knowledge for a better world 11

3. Spontaneous uptake of oil: The core, now saturated with water at residual oil saturation, is placed in an Amott cell and surrounded by oil. The oil is spontaneously taken up and water is displaced. The volume of Rubber tube Oil Core sample water displaced is measured; Vw 1. Displaced water 4. Forced displacement of water: The core is removed from the cell after equilibrium is reached, and remaining water in the core is forced out by displacement in a flooding rig. The volume of water displaced is measured; Vw 2 Core holder with rock sample Oil index: Knowledge for a better world Water 12

Amott-Harvey wettability index VO 1 = volume of oil produced during water imbibition VO 2 = volume of oil produced during water flooding VW 1 = volume of water produced during oil “imbibition” VW 2 = volume of water produced during oil flooding rw = water index ro = oil index. WI = 1. 0 completely water wetting WI = 0. 0 neutral WI = - 1. 0 completely oil wetting Knowledge for a better world • Empirical test based on some theoretical reasoning. • The test steps are difficult to perform at reservoir pressure and temperature. 13

Measurement on Core Samples: Centrifuge- or USBM-method • The centrifuge method for determining wettability is based on a correlation between the degree of wetting and the areas under the capillary pressure curves. • The method is often called the USBMmethod (USBM is abbreviation for United States Bureau of Mines). Photo: Georg Voss Knowledge for a better world 14

Capillary pressure Pressure in the non-wetting phase minus the pressure in the wetting phase. For a two fluid system the saturation in a core plug rotating in a centrifuge looks like this: Knowledge for a better world

Measurement on Core Samples: Centrifuge 1. A core sample is saturated with brine. 2. The core is then placed in a centrifuge core holder. 3. The core holder is filled with oil and rotated at a certain speeds to obtain the primary drainage curve. 4. The core is placed in an inverted core holder filled with brine. The brine is allowed to spontaneously imbibe into the core. Then the core is centrifuged at incremental steps. 5. The core is placed in a core holder filled with oil and the secondary drainage curve is obtained. 6. The areas under the two curves are determined Knowledge for a better world From N. R. Morrow (JPT, Dec. 1990)

Knowledge for a better world From N. R. Morrow (JPT, Dec. 1990) 17

Measurement on Core samples: Centrifuge • The logarithm of the area ratio is defined as the USBM-wettability index; WIUSBM = log(A 1/A 2). • The relative wetting tendencies of the liquids in a porous medium and the distribution of pore sizes determine the shape of the capillary-pressure curves. • In general, water-wet systems should have a larger area in the water-displaced-by-oil curves (area A 1) than the area under the oil-displaced-by-water curves (area A 2). • Therefore, the logarithm of the area ratio for the water-wet system is greater than zero. Conversely, the area ratio is less than unity for oil-wet systems and the logarithm of the ratio is negative. WIUSBM = ∞ WI = 0 WIUSBM = - ∞ Knowledge for a better world completely water wetting neutral completely oil wetting

Final remark • Todays methods for measuring the wettability of a crude oil/brine/rock system have a weak theoretical basis, are time consuming to perform and the results are questionable. • In WP 6 in Pore. Lab we will study wettability and hopefully be able to introduce a new and better measurement method for crude oil/brine/rock systems. Knowledge for a better world