**4. Diffusion and mixing of fracturing and hydrocarbon reservoir fluids**

Hydrocarbon reservoirs are often multicomponent and multiphase. This means that in their natural state, there are variations in composition of reservoir fluids, occurring longitudinally and/or vertically. Key drivers of changes in reservoir

#### **Figure 1.**

 *Pressure gradient of multicomponent reservoir fluids without capillary effect. P V, P L and P W are the vapour, liquid and water pressure, respectively [32].* 

fluid composition are gravity, capillary forces and temperature gradients [31]. Gravity and capillary effects are major factors influencing variations in composition with depth ([32, 33]). Due to gravity, reservoir liquid hydrocarbons lie atop aquifers, which is a reflection of the differences in density between the two fluids (hydrocarbons are less dense and immiscible in water). In terms of hydrocarbons, the gas phase lies above the liquid phase, and their individual pressure gradient is dependent on their corresponding densities [32]. An idealised form of this, ignoring capillary effects, is shown in **Figure 1**.

Discounting capillary action renders the illustration in **Figure 1** unrealistic for formation rocks which are porous and therefore composed of pore spaces. Capillary forces due to surface tension within the pores act in opposition to external forces such as gravity. In addition, reservoirs are likely to contain a mixture of multicomponent fluids at the different phases (gas and liquid), such as the occurrence of pockets of heavy hydrocarbons or injected fluids within the predominant fluid type. This will change the composition with depth in any or both of the following ways: firstly, capillary forces prompt the occurrence of a transition region, which is an overlap consisting of two or more phases instead of the sudden change shown in **Figure 1** and, secondly, the compositional gradient of the reservoir fluid is altered because of the changes in its components. A modified pressure gradient profile which also accounts for capillarity and compositional variation is given in **Figure 2**.

#### **Figure 2.**

*Pressure gradient of multicomponent reservoir fluid with the combined effects of gravity, capillarity and variation in composition [32].* 

*Introductory Chapter: Developments in the Exploitation of Unconventional Hydrocarbon… DOI: http://dx.doi.org/10.5772/intechopen.86625* 

 Temperature gradients in formations introduce an extra dimension to the behaviour of reservoir fluids. The effect of variations in temperature induces convection and thermal diffusion. For small temperature gradients collinear with the gravity vector, convection can be neglected [31]. Thermal diffusion, also known as 'Soret effect', is the separation of a non-convective mixture due to a thermal gradient [34]. In other words, there is movement of material during the occurrence of thermal gradients resulting in corresponding concentration gradients of the constituents of the fluid mixture. This process is measured by the thermal diffusion coefficient, α. Thermal diffusion can have a substantial impact on variations in composition of reservoir fluids [34, 35]; it may increase or attenuate compositional variation vertically and horizontally.

The mixing of injected fracturing fluids with in situ and/or other fracturing fluids affects the constituent composition and variation of reservoir hydrocarbon fluids and the stimulation process during enhanced oil/gas recovery. The introduction of alien fluid(s) into the formation sets off a mixing mechanism and displacement of resident fluids, which improve hydrocarbon production. Controlling factors include, but not limited to, the injected/resident fluid properties (e.g. rheology, density and viscosity), formation rock properties, the reservoir condition (thermal gradient, pre-existing fluid compositional variation) and other drivers such as capillarity and diffusion. The practicability of this process involving a threephase fluid system (scCO2-brine-oil) is demonstrated in Jiménez-Martínez et al*.* [36], where supercritical CO2 (as an injection fluid) is used to restimulate an oil-wet shale formation containing brine and hydrocarbon as the major resident fluids.
