Structure and Mobility of Irreducible Oil in Anthropogenic Changed Formations

Irreducible oil (IO) is researched as hydrodynamic system, consisting of separate types. IO of micro- and macrolevel, connate and conventionally mobile IO were defined. We identified that such technological factors as pressure gradient, wettability, surface tension, etc. effect IO significantly. Irreducible oil saturation (IOS) is one of the most important parameter characterizing development efficiency. Traditionally IOS was associated with formation microconstruction and considered through displacement efficiency using its relationship with filtration capacity properties of formation. IOS was considered as one of producing formation properties. Technology influence on final oil recovery used to be determined through volumetric efficiency. It was determined that conventionally mobile irreducible oil becomes movable under the critical pressure gradient to capillary pressure ratio. When the mobility threshold is reached the values of IOS depend on technological displacement condition. After achieving the second threshold value almost all conventionally mobile oil is withdrawn and only connate oil remains in formation. Present geo-technological models do not take the IOS influence of these factors into consideration. Therefore the dynamic models of IOS were developed with the attention paid on threshold mobility character, operating practices and mechanisms of IOS formations. Distribution research of IOS in croswell area was based on special hydrodynamic models. Results of investigation technology effect on IOS (e.g. well spacing system, pattern arrangement, disturbances and modernization of designed system) are presented. As shown, the compound anthropogenic heterogeneity of IOS is formed in crosswell area even for homogeneous formations. Depending on well spacing system IOS can be heterogeneous regarding displacement efficiency and phase permeability. We consider the possibility of additional IO recovery using intensive technologies (flow control, sidetracks, horizontal wells). Taking different well spacing systems, analysis of relative influence of formation microconstruction and of technological factors on IOS was given.