EOR and Unconventional Resources

Chemical EOR mechanisms

Chemical enhanced oil recovery improves displacement by changing the interfacial tension, the wettability of the rock, or the mobility of the injected fluid. Alkaline flooding lowers the oil-water interfacial tension and can reverse rock wettability, which helps mobilise residual oil. Polymer flooding does not reduce water viscosity; the polymer dissolved in the injected water raises the water viscosity and lowers its mobility. For water displacing oil, the mobility ratio is M = \dfrac{k_{rw}/\mu_w}{k_{ro}/\mu_o}, and a polymer that increases \mu_w reduces M. A lower mobility ratio stabilises the displacement front, suppresses viscous fingering, and improves sweep efficiency and oil recovery. Alkali is delivered as anhydrous solid and must be diluted on site. Because dissolving sodium hydroxide in water is strongly exothermic, a cold utility supply is needed, and personal protective equipment and adequate ventilation are essential against caustic burns and aerosols.

Heavy oil viscosity reduction

Heavy oil is viscous because of its high asphaltene and resin content and its low hydrogen-to-carbon ratio, so its viscosity must be lowered or its composition upgraded before pipeline transport. Vis-breaking is a mild thermal cracking process that breaks large molecules to reduce viscosity, and coking thermally converts heavy residue into lighter liquid products. Hydrotreating adds hydrogen to saturate the molecules and removes sulphur, nitrogen and metals, which lowers density and viscosity. Drying removes free water and salts as a pretreatment step, but it does not change the viscosity of the hydrocarbon phase, so it is not a viscosity-reduction process. In reservoirs, heavy oil viscosity is reduced by heating, as in huff-and-puff steam stimulation. For steady radial flow, the rate is q = \dfrac{0.00708\,kh\,\Delta p}{\mu\ln(r_e/r_w)} in field units. When the near-wellbore region has a different viscosity, the radial resistances in series add as \sum_i \mu_i \ln(r_{i+1}/r_i), so a low-viscosity steam-swept zone around the well raises the rate in inverse proportion to the total resistance.

Coalbed methane recovery

Coalbed methane is stored mainly as gas adsorbed on the internal surfaces of coal, so producing it requires desorption. Primary production is commonly achieved by depressurisation, in which formation water is pumped out through vertical or horizontal wells to lower the reservoir pressure, allowing methane to desorb and flow to the wellbore. Enhanced recovery injects gases such as CO_2 or N_2. The controlling property is adsorption affinity, not molecular kinetic energy: CO_2 binds to coal more strongly than CH_4, so it preferentially adsorbs and displaces the adsorbed methane, whereas N_2 binds more weakly and is less effective for this purpose. Underground coal gasification and underground combustion convert the coal and destroy the methane, and open-cast mining removes the seam, so none of these recovers the methane in situ.

Well pattern configurations

A well pattern is an arrangement of injectors and producers that controls the areal sweep. In an inverted five-spot, a single injector sits at the centre and producers occupy the corners of the square. A regular seven-spot pattern places one central producer among six injectors at the corners of a hexagon, with each injector shared by neighbouring patterns. A staggered-line drive arranges injector and producer rows in an offset layout so that each producer lies between injectors. Crestal injection places injectors at the structural crest of the reservoir, not along the periphery. In a five-spot pattern, each producer is surrounded by four corner injectors that are shared with adjacent patterns, so in an infinite areal pattern the number of producers equals the number of injectors and the producer-to-injector ratio is 1.

Hydraulic fracturing proppants

Hydraulic fracturing creates fractures by injecting fluid at a pressure above the formation fracture gradient. When the injection pressure is released, the fractures tend to close, so proppant particles such as sand or ceramic beads are carried into the fracture to hold it open and keep a conductive path for hydrocarbon flow. Proppants do not change the viscosity or density of the fracturing fluid, nor the viscosity of the crude oil in the reservoir. Proppant carrying capacity depends mainly on fluid viscosity, because higher viscosity reduces the settling velocity of suspended particles. Cross-linked gels, whose polymer networks give a high apparent viscosity, therefore transport proppant far better than low-viscosity fluids such as pentane, slickwater or untreated formation brine. Slickwater relies on high flow velocity rather than viscosity to carry proppant.

Micellar slug design

A micellar or microemulsion slug is a volume of surfactant-rich fluid injected ahead of a polymer drive to displace residual oil. The slug volume is usually expressed as a fraction of pore volume, and the surfactant mass in the slug equals the slug volume multiplied by the surfactant concentration. Part of the injected surfactant is lost by adsorption onto the rock. The fraction adsorbed equals the adsorbed mass divided by the injected mass, where the adsorbed mass is the adsorption density in mg per gram of rock times the rock mass, and the rock mass per unit bulk volume is (1-\phi)\rho_{rock}. For a surfactant flood, the oil displaced equals the drop in oil saturation from the waterflood residual to the surfactant-flood residual, so the ratio to original oil in place is E_v(S_{orw}-S_{orc})/S_{oi}, where E_v is the volumetric sweep efficiency. Pore volume and formation volume factor are common to numerator and denominator and cancel in this ratio.

Total acid number of crude oil

The Total Acid Number (TAN) measures the acidic content of a crude oil, which indicates its potential to form in-situ surfactants when reacted with alkali in chemical EOR. TAN is defined as the milligrams of potassium hydroxide required to neutralise one gram of crude oil, so its unit is mg KOH/g. Sodium hydroxide and calcium hydroxide are bases but are not the reference reagent in the TAN definition, and a neutral salt such as NaCl cannot neutralise acidic components, so it cannot define an acid number.

Water-alternating-gas injection

Water-alternating-gas (WAG) injection alternates slugs of water and gas to control gas mobility and improve sweep. Water in the alternating slugs occupies part of the pore space and reduces the relative permeability to gas, which in turn lowers the gas mobility, defined as k_{rg}/\mu_g. Lower gas mobility improves the mobility ratio between the injected gas and the oil, which limits gas override and channelling. The purpose of WAG is therefore to reduce both the relative permeability and the mobility of the gas, not to enhance them.