Fall 2012 Lecture 4 Petrophysics

Reservoir Fluids

Crude Oil Definition A mixture of hydrocarbons that existed in the liquid phase in natural underground reservoirs and remains liquid at atmospheric pressure after passing through surface separation facilities. http://www.am765.com/zt1/ztfl/ypzt/szx/pm/200711/t20071116_306032.htm

Chemical Composition of Petroleum paraffins (CnH2n+2) naphthenes (CnH2n) aromatics (C6H5 + CnH2n+2) hydrocarbons oxygen compounds sulfur compounds nitrogen compounds others crude oil heterocompounds petroleum dry gas (methane) ethane (C2H6) propane (C3H8) butane (C4H10) natural gas (paraffins) wet gas

General Categories of Reservoir Hydrocarbons Reservoir fluid Surface appearance API gravity (degree) Dry gas Colorless gas Wet gas Colorless gas + small amount of clear liquid 60~70 Condensate Colorless gas with significant amounts of light-colored liquid 50~70 Volatile oil Brown liquid with various yellow 40~50 Black oil Dark brown to black viscous liquid 30~40 Heavy oil Black, very viscous liquid 10~25 Tar Black substance

Property of Reservoir Fluids Formation Volume Factor (FVF) Definition Bo: Oil and dissolved gas volume at reservoir conditions divided by oil volume at standard conditions. Bg: Gas volume at reservoir conditions divided by gas volume at standard conditions.

Property of Reservoir Fluids Density Typical value ≈ 800 kg/m3 (black oil) Common unit: kg/m3, lb/ft3 (Courtesy of Anton Paar)

Property of Reservoir Fluids Viscosity Definition: A property of fluids that indicates their resistance to flow, defined as the ratio of shear stress to shear rate. Typical value ≈ 0.2 ~ 20,000 cp Common unit: Poise, cp, Pa•s, mPa•s

Property of Reservoir Fluids Compressibility Definition: the relative change in fluid volume related to a unit change in pressure. Common unit: MPa-1, psi-1

Properties of Reservoir Rocks Porosity ( ) casting film (铸体薄片） http://lonestarsecurities.com/images/porosity.jpg

Typical Porosity Values

Typical Porosity Values Upper limit with uniform bead packing: 48% Very high porosity sandstone: 35% Good porosity: 20% ~ 30% Tight gas porosity: 8% ~ 12% Shale gas porosity: 5% ~ 10%

Primary and Secondary Porosity Important processes sedimentation, cementation, recrystallization, solution, weathering, fracturing For sandstone reservoir primary porosity: intergranular porosity secondary porosity: dissolution porosity, fracture porosity, microporosity (Note: microporosity exists as small pores, <1um, usually associated with clay minerals.) For carbonate reservoir two common porosity types: fracture porosity (0.01~0.1%), vugular porosity (up to 10%)

Porosity Types in Carbonate Reservoirs

Field Example for Different Types of Porosity Inter & Intra granular porosity

Field Example for Different Types of Porosity Fracture porosity Vug porosity

Properties of Reservoir Rocks Permeability ( ) The ability or measurement of a rock formation to conduct fluids Units: 1 Darcy = 1,000 millidarcy (md) = 9.8692E-16 m2 http://www.liteoilinvestments.com

Porosity v.s. Permeability Roughly log relation, determined by rock type http://www.beg.utexas.edu/indassoc/rcrl/rckfabpublic/poros-perm.htm

Properties of Reservoir Rocks Saturation (s): the fraction of the pore space occupied by some fluid (water, oil, or gas). http://www.slb.com/content/services/evaluation/petrophysics/resistivity/rt_scanner.asp

Properties of Reservoir Rocks Rock compressibility (cf) The fractional change in volume of the solid rock material (grains) with a unit change in pressure.

Fluid Content of the Reservoir Reservoir fluids: “Oil + water + gas” Interstitial water (connate water): 间隙水（原生水） Water trapped in the pores of a rock during formation of the rock. Irreducible water：束缚水 The lowest water saturation, Swi, that can be achieved in a core plug by displacing the water by oil or gas. The state is usually achieved by flowing oil or gas through a water-saturated sample. Aquifer: 水体 A considerable volume of water underlines the oil in the same sedimentary bed it is referred as the “aquifer” and being under pressure also, it contributes to the total energy of the reservoir. Dissolved gas：溶解气

Fluid Content of the Reservoir Saturated reservoir：饱和油藏 If the reservoir cannot dissolve more gas under these particular pressure and temperature conditions. Unsaturated reservoir：未饱和油藏 If the reservoir could dissolve more gas at the same pressure and temperature. Gas cap：气顶 In many cases there can be more gas in the reservoir than the oil is capable of holding in solution. This extra gas being lighter than the oil will have formed a “gas cap” above the oil accumulation.

Definition and Causes of Wettability Definition: Wettability is a term used to describe the relative attraction of one fluid for a solid in the presence of other immiscible fluids. Microscopic fluid distribution Residual oil saturation water-wet oil-wet neutral wet mixed wet

Definition and Causes of Wettability

Contact Angle Wettability may be represented by the contact angle formed among fluids and a flat solid surface or the angle formed between the fluids’ interface and a glass capillary tube.

Worldwide reservoir wettability surveys (Liquid Hydrocarbon Bearing reservoirs)

Worldwide reservoir wettability surveys (Liquid Hydrocarbon Bearing reservoirs)

Wettability Influence on Multiphase Flow The microscopic distribution of fluids in a porous medium is greatly influenced by the degree of rock preferential wettability.

Wettability Influence on Multiphase Flow Fig. “Channel flow”diagram—Wetting phase driving non-wetting phase

Fluid distribution for oil-wet rocks during water injection

Relative Permeability

Absolute Permeability The resistance to fluid flow existing in a porous media when it is the only phase present.

Relative Permeability The resistance to fluid flow existing in a porous media when it is in the presence of other mobile or immobile, immiscible fluids.

Impact of Wettability on Relative Permeability to Water and Oil Water Wet High Frictional Resistance, Low Relative Permeability to Water

Impact of Wettability on Relative Permeability to Water and Oil Oil Wet Lower Frictional Resistance, Higher Relative Permeability to Water

Wettability Influence on Multiphase Flow Relative permeability become progressively less favourable to oil production as a rock becomes less water-wet. The residual oil saturation increases as a rock becomes more oil-wet.

Relative Permeability Nomenclature Oil/Gas Primary Endpoint Perm Values Permeability or Relative Permeability Water Fluid Saturation

Relative Permeability Nomenclature Fluid Saturation Increasing Relative Permeability Curves Permeability or Relative Permeability Fluid Saturation

Relative Permeability Nomenclature Terminal Endpoints Permeability or Relative Permeability Fluid Saturation

Relative Permeability Nomenclature Crossover Point Permeability or Relative Permeability Fluid Saturation

Relative Permeability Water Wet Oil Wet Relative Permeability - Fraction Water Saturation - Fraction

Wettability Influence on Multiphase Flow Strongly water-wet 1) Oil is trapped in larger pores, water advances along the wall of the rock. 2) The effective permeability to the non-wetting phase at irreducible water saturation is approximately equal to the absolute permeability of the rock. Strongly oil-wet 1) Water is in larger pores. 2) The effective permeability to oil at irreducible water saturation is greatly reduced by the water droplets in the larger pores.

Effects of Saturation States on Permeability Relative permeability curves in water wet reservoirs

Effects of Saturation States on Permeability If Sw > Swi (Assume water is the wetting phase) Water is mobile, kw increases, but kwr < kor. Reason 1: the adhesion force between the solid surface and wetting fluid; Reason 2: the greater tortuosity of the flow path for the wetting phase. When Sw increases, the non-wetting phase (oil) breaks down and forms a discontinuous phase at the critical non-wetting phase saturation. This is called an insular state of non-wetting phase saturation.

Effects of Saturation States on Permeability Residual non-wetting phase saturation 非润湿相残余饱和度（如残余油饱和度） As the non-wetting phase saturation reduces, the network for this phase breaks down and becomes discontinuous; the remaining stationary islands of the non-wetting phase cannot be displaced at pressure gradients encountered in hydrocarbon reservoirs. Irreducible wetting-phase saturation 润湿相束缚饱和度（如束缚水饱和度） As the wetting phase saturation decreases, the network through which this phase flows breaks down and becomes discontinuous and immobile.

Effects of Saturation on Relative Permeability Saturation is a term used to describe the relative volume of fluids in a porous medium. Low saturation (wetting phase)  pendular ring (液环)

Reservoir Heterogeneity Although oil reservoirs are characterized as porous media with certain porosities and permeabilities, they are almost never homogeneous beds with constant properties. Generally, there are numerous strata with wide-ranging properties. The permeability variation and fractures can have a profound effect on the flow of fluids in a reservoir and thereby influence oil recovery.

Reservoir Heterogeneity 岩心铸体薄片X100摄像 岩心电镜图片X200摄像 ESEM2020扫描电镜

Reservoir Heterogeneity

Types of Reservoir Heterogeneities Areal variations Vertical variations Reservoir-scale fractures It is obvious that the reservoir may be nonuniform in all intensive properties such as permeability, porosity, pore size distribution, wettability, connate water saturation and crude properties.

Types of Reservoir Heterogeneities The heterogeneity of reservoirs is, for the most part, dependent upon the depositional environments and subsequent events (compaction, solution, dolomitization, and cementation), as well as on the nature of particles constituting the sediment. Lateral similarity At an elevation corresponding to a given deposition period, the same basic particle size range should exist over wide areal expanses. Vertical variation The variation in rock properties with elevation would be largely due to differing depositional environments or to segregation of differently sized or constituted sediments into layers, or to both.

Types of Reservoir Heterogeneities Sandstone reservoir The properties (porosity and permeability) depend on the nature of the sediment, on the environment of deposition, and generally on subsequent compaction and cementation. Carbonate reservoir Related to selective solution, replacement, recrystallization, and dolomitization, etc.

Waterflooding Recovery

Preferential Flow Channel When water or other fluids are injected under pressure, they will seek the path of least resistance to the point of lowest pressure, which is generally the producing well. Because the high permeability zones and the fractures offer the least resistance to flow, most of the injected fluid follows this path. In doing so, most of the oil remaining in the lower permeability zones is by passed.

Sweep Efficiency Vertical sweep efficiency (invasion efficiency) It is defined as the cross-sectional area contacted by the injected fluid divided by the cross-sectional area enclosed in all layers behind the injected fluid front. Areal sweep efficiency The areal fraction which has been contacted at the time of breakthrough of injected fluid is defined as the areal sweep efficiency.

Sweep Efficiency Volumetric sweep efficiency A measure of the three-dimensional effect of reservoir heterogeneities. It is the product of the pattern areal sweep and vertical sweep. Stated another way, the volumetric sweep efficiency is the pore volume of the reservoir contacted by the injected fluid divided by the total pore volume.

Well Pattern Effects on Sweep Efficiency Vertical sweep efficiency is a function of reservoir characteristics alone, while areal sweep efficiency is a function of reservoir characteristics and well locations. The geometric pattern for injection and producing wells affects the areal sweep efficiency. Improper placement of wells can lower this efficiency even in the absence of detrimental reservoir heterogeneities.