Irreducible Saturation Research Articles

HEAT AND MASS TRANSFER DURING SAPWOOD DRYING ABOVE THE FIBRE SATURATION POINT

ABSTRACT Pine sapwood was dried in an air convection kiln at temperatures between 60-80 °C. Temperature and weight measurements were used to calculate the position of the evaporation front beneath the surface. It was assumed that the drying during a first regime is controlled by the heat transfer to the evaporation front until irreducible saturation occurs. Comparisons were made with CT-scanned density pictures of the dry shell formation during initial stages of drying of boards. The results indicate a receding evaporation front behaviour for sapwood above approximately 40-50% MC when the moisture flux is heat transfer controlled. After that we finally reach a period where bound water diffusion is assumed to control the drying rate. The heat transfer from the circulating air to the evaporation front controls the migration flux. In many industrial kilns the heating coils therefore have too small heat transfer rates for batches of thin boards and boards with high sapwood content.

A high-temperature drying model for softwood timber

The high-temperature drying of a softwood board with relatively high permeability was modelled on a one-dimensional basis. The moisture-transport mechanism between fibre saturation and irreducible saturation was elucidated, and moisture flow within the thin layer of damaged cells at the sawn surfaces of the timber board was examined. The predicted temperature and average moisture-content profiles with respect to time were in good agreement with some experimental data from the literature. Comparison between the experimental and predicted within-board moisture-content profiles illustrated the importance of accounting for the orientation of the growth rings with respect to the drying surfaces and the effect of density variations on liquid permeability at high core moisture contents.

DRYING OF POROUS MATERIALS AT LOW MOISTURE CONTENT

ABSTRACT Drying experiments were conducted using packed beds of glass beads with initial moisture content near or slightly above the irreducible moisture content. The objective was to validate the vapor phase diffusion coefficient determined in earlier experiments and to quantify the effect of temperature gradient on vapor phase diffusion in the presence of irreducible saturation. The resulls indicate that for isothermal drying the enhancement factor, β, which is the ratio of the diffusion coefficient in the packed bed to that in free space, is slightly less than unity. This is consistent with previous results. However, the results for non-isothermal drying show little effect of temperature gradient for temperature gradients either assisting the concentration gradient or opposing the concentration gradient. This is contrary to the traditional thinking about enhanced vapor diffusion originally proposed by Philip and deVries (1957).

Remediation of Water And Hydrocarbon Phase Trapping Problems In Low Permeability Gas Reservoirs

Abstract Aqueous and hydrocarbon phase traps can occur in porous media when water- or oil-based fluids come into contact with a formation which exhibits a "subirreducible" initial liquid saturation of the phase. of interest. This commonly occurs with waterbased fluids in many low permeability desiccated gas bearing formations and in depleted conditionsinrich gas retrograde condensate reservoirs. This paper documents how phase traps are induced by direct displacement, countercurrent imbibition or depletion effects, and presents techniques for diagnosing whether a reservoir is a candidate for an aqueous phase trapping problem. Techniques to minimize problems with aqueous and hydrocarbon phase trapping are reviewed, followed by discussion of methods to reduce or remove the effect of existing phase traps, such. as increased drawdown, alteration of 1FT, alteration of pore geometry or direct removal methods. A brief discussion of laboratory techniques used to screen the optimum processfor selection are also presented. Introduction The phenomena of permanent entrainment of extraneous or in situ generated aqueous- or hydrocarbon-based liquids in porous media has been documented in the literature as a mechanism for significant permeability impairment in low permeability intercrystalline sandstone and carbonate formations (1–5). Phase traps normally occur when a water- or hydrocarbonbased fluid is either forced or imbibed into porous media with a subirreducible water or hydrocarbon saturation (i.e., at a saturation less than the irreducible liquid saturation given the geometry, wettability, and capillary mechanics of the system under consideration). Subirreducible hydrocarbon saturations are common in rich gas retrograde systems existing in a sub-dewpoint condition, or in mature gas fields which may have migrated into previously oilsaturated strata. Subirreducible water saturations in low permeability gas reservoirs are also quite common. The mechanism for the establishment of a subirreducible water saturation in a low permeability gas-bearing reservoir is the subject of some controversy. The dominant mechanism is thought to be desiccation motivated by a large regional migration of gas under conditions of increasing temperature and pressure through a given reservoir area over a long period of geological time. The basic mechanism of a phase trap is created by the relative permeability effect associated with an increase in the immobile water or hydrocarbon saturation. This phenomena is illustrated on a pore scale in Figure 1 and from a mechanistic point of vie\v as Figure 2. The severity of the phase trap is strongly influenced by:The magnitude of the difference between the "initial" and final trapped "irreducible" liquid saturation which exists in the porous media. The greater this difference, the larger the adverse relative permeability effect and the greater the potential reduction in permeability.The configuration of the gas or oil phase relative permeability curves at low liquid saturation levels. The more adverse the configuration of these curves (i.e., the more convex the relative permeability curve), the more significant the reduction in permeability for a given increase in trapped liquid saturation.The depth of invasion of the trapped phase. The greater the volume of fluid lost and deeper the invasion, the more difficult the mobilization of this fluid becomes.

A Modified Leverett J-Function for the Dune and Yates Carbonate Fields: A Case Study

Effective medium theory (EMT) has been used to model capillary pressure (Pc) as a function of water saturation (Sw) in porous media. The EMT model results show that both the magnitude and profile of the Pc − Sw profile are strongly affected by changing the pore-size distribution parameters and the rock pore geometry. Tortuosity, which is a function of these parameters, is used in developing a drainage capillary pressure model based on data from the Dune and Yates fields. Capillary pressure is normalized with respect to an average pore radius that includes tortuosity. To minimize the uncertainty caused by the contact angle term, only helium/water capillary pressure curves were considered. The wetting-phase saturation is normalized with respect to an asymptotic irreducible wetting-phase saturation. For the Dune and Yates fields, the development of this type of dimensionless model may be useful as an input for reservoir simulation studies.

The prize; what's possible?

Recovery factors for comparable reservoirs are often from 10 to 20% higher in the USA than elsewhere. The reason is the relatively low cost of drilling and work-over combined with the high revenue per barrel for the producer in the States. This implies that elsewhere the scope for additional recovery will frequently be of the same order as the noted differences. Another motivation for re-evaluation of older fields is the realization that irreducible oil saturations measured on core plugs in the past were generally much too high. Consequently, the assumed sweep efficiencies were much too high also, resulting in little incentive for infill drilling or recompletion. A number of factors stimulate the re-evaluation of remaining reserves. Reliable through-casing logging makes the poorly swept zones visible. Detailed structural definition is often possible with 3D seismic. New drilling techniques, like short radius horizontal side tracks, allow economic recompletions of old wells.

Moisture conditions in coated exterior wood Part 4: Theoretical basis for observed behaviour. A computer modelling study

A theoretical investigation of moisture movement in pine sapwood was undertaken using two different models of behaviour. Firstly a model was developed using a single diffusion process; this model predicted levels of water absorption and moisture content which were in broad agreement with experimental results. However, this approach proved to be over-simplistic, and predicted a more rapid penetration of moisture than was observed in practice. The model also failed to predict the deceleration in absorption which was recorded in laboratory tests. Consequently, the predictions of the model were of a more uniform moisture distribution than was consistent with experience. A development of the model, to include flow mechanisms appropriate for the level of moisture content, gave more realistic predictions. The improved model is based on different flow mechanisms in three distinct regions of moisture content. Below fibre saturation point (FSP), a diffusion mechanism is assumed for the calculation of moisture flow. Between FSP and a threshold, termed the irreducible saturation point (ISP), flow takes place through the cell wall material. Above the ISP, a continuous path is established for moisture movement from cell to cell through the bordered pits. The improved model proved capable of predicting levels of moisture uptake and moisture content, and of predicting the moisture gradients observed in practice. It also provided an explanation of some of the observed patterns of wood moisture movement reported in earlier parts of this series of papers. In particular it explained why, when water is absorbed by coated timber, the rise in surface moisture content appears not to exceed FSP. Additionally it provided an understanding of the continuing penetration of moisture in depth during the early stages of drying. This is a significant conclusion, because it imposes a limitation, in principle, on the performance of high permeability wood coatings.

The effect of temperature on capillary pressure‐saturation relationships for air‐water and perchloroethylene‐water systems

The temperature dependence of capillary pressure‐saturation relationships was measured for air‐water and perchloroethylene‐water systems in silica sand. Changes in capillary pressures, irreducible water phase saturations, and residual nonwetting saturations with temperature were determined. Relationships for temperature dependence of contact angle and interfacial tension were incorporated into the van Genuchten [1980] model and fitted to the data. Capillary pressures at constant degrees of saturation decreased as temperature increased. Hysteresis decreased, irreducible water saturations increased, and residual nonwetting saturations decreased as temperature increased. The magnitude of the change in capillary pressures could not be explained by the temperature dependence of wetting‐nonwetting interfacial tensions alone. Derived parameters for the temperature dependence of the contact angle predicted an increase of contact angle of roughly 45°–50° for air‐water and perchloroethylene systems with a temperature increase from 20° to 80°C, while literature studies suggest that contact angles should decrease with increasing temperature. It was concluded that the parametric relationship for temperature effects incorporated into the van Genuchten [1980] model fit the data well, but other effects in addition to changes in interfacial tension and contact angle played a role in the temperature dependence of capillary pressure‐saturation relationships.

Irreducible fluid saturation determined by pulsed field gradient NMR

A new laboratory procedure, using a pulsed field gradient (PFG) NMR technique, for measuring the relative volumes of movable and nonmovable fluid in cores is reported. The saturations determined are in qualitative agreement with values obtained from mercury porosimetry and centrifuged gravimetric saturation measurements. An advantage of the PFG NMR method is that it is fast and does not require exchange of fluids, i.e., progressive desaturation. This may be important if wettability changes are a concern or for measurements on unconsolidated cores. The bound volume index, BVI, of some core plugs was additionally determined by CPMG NMR which was recently established as a down-hole measurement with the new generation of NMR well logging tools. Since PFG NMR is not sensitive to surface relaxivity, this technique can be used as a quick and independent check on the BVI values obtained by CPMG NMR.

Underbalanced Drilling And Formation Damage-Is It a Total Solution?

Abstract Underbalanced drilling has been utilized as a technique to minimize damage- due to whole mud, fluid filtrate and solids invasion into high permeability" and fractured formations and formations which exhibit a high degree of sensitivity to invaded fluids. While it is generally generally accept that the process of underbalanced drilling reduces the potential for formation damage, the possibility of invasive formation damage ‘still’ exists in certain reservoirs where the initial saturation (either hydrocarbon or water) is substantially less than the irreducible saturation of the phase which is being utilized in the drilling or completion process. This situation could conmonly occur when drilling in an underbalanced mode in gas reservoirs that are in a desiccated state and exhibit abnormally low initial water saturations, or in oil and gas reservoirs in a portion of the reservoir significantly, above the oil-water or water-gas transition zone. Although dynamic fluid flow is occurning from the formation during the drilling process, the circulating drilling fluids are in continual contact with the formation face. At the low initial liquid saturations which exist in these reservoirs, there is an ability for strong countercurrent spontaneous imbibition effects which can, in some situations counteract pressures that are far greater than the apparent underbalance pressure which is present during the drilling or completion process. If the formation contains potentially sensitive materials .sucl;1 as deflocculatable fines or swelling clays, exhibits the potential for the formation of in situ emulsions or the possibility of permeability -reductions due to the establishment of near wellbore aqueous phase traps, these phenomena could occur and cause potential reductions in productivity of the well. The majority of underbalanced drilling fluid systems are not designed with ultra-low fluid loss in mind as extensive mud losses to the formation are not contemplated. However, the fact that a stable filter cake is not developed during underbalanced drilling, due to continual inflow from the formation face, could increase the severity of losses to the formation if underbalanced conditions are not maintained 100% of the time during drilling and completion. Overbalanced 'pulses may occur due to mud pulsed logging programs, bit trips or mechanical problems which may occur during the drilling process of most horizontal wells. This results in the majority of underbalanced horizontal wells being overbalanced for some fraction of the time during the drilling and completion process. A discussion of mechanical damage due to near surface glazing effects is also presented as a possible mechanism of damage which can occur during some underbalanced drilling operations. The paper elaborates on potential damage scenarios based on the results of specific laboratory tests which have been conducted and reviews a variety of new techniques 'which have been designed to evaluate and pre-screen fluids and procedures prior to costly and possibly ineffective implementation of an underbalanced drilling operation in the field.

Secondary migration of hydrocarbons in inclined layers

SUMMARY The 1-D models of segregated two-phase flow in reservoir engineering are utilized to model lateral secondary migration of hydrocarbons in slightly inclined and permeable layers. It is assumed that the layers are sealed below and above by low-permeability strata. The flow is then characterized by a gravity number and a mobility ratio when capillary forces are neglected. For buoyancy-driven migration, gravity numbers (much) larger than 1, these models imply that the Darcy velocity of the flow is only given by the buoyancy force due to the density difference between water and oil. For gravity numbers larger than 1, the thickness of the hydrocarbon zone below the sealing top is proportional to the inverse gravity number. Furthermore, if the oil flow in the layer is only a small fraction of the total flow, the height of the hydrocarbon zone is also proportional to this fraction. It is therefore concluded that in buoyancy-driven migration with large gravity numbers, where the hydrocarbon part of the total flow is small, only a small fraction of the upper part of the layer is saturated with oil. Therefore, only a little oil may be lost due to irreducible saturation, and long-range migration is possible.

Reductions In the Productivity of Oil And Low Permeability Gas Reservoirs Due to Aqueous Phase Trapping

Abstract Many hydrocarbon bearing reservoirs exhibit the potential for significant productivity reductions due to adverse relative permeability effects associated with the retention of invaded aqueous fluids. These fluids could include-water-based drilling mud filtrates, completion fluids, fracture fluids, workover fluids, kill fluids or stimulation fluids (including-spent acid). This paper identifies potential mechanisms behind phase trapping and identifies particular reservoir types which tend to be susceptible to this type of formation damage, most notably low initial water saturation gas reservoirs and strongly oil-wet oil reservoirs. Laboratory techniques to investigate the severity of aqueous trapping and various remedial techniques are described, and two field case studies illustrating the potential for permeability impairment due to invasive aqueous trapping are presented. One case study describes a series of wells completed in the Paddy formation and the second in the Cadomin formation in the Deep Basin area of central Alberta (both gas producing zones). Laboratory case studies documenting the phenomenon of aqueous, phase trapping in strongly oil-wet porous media are also presented. Introduction Oil and gas bearing formations are potentially susceptible to many different types of formation damage(1–6). In this paper we are exclusively concerned with damage associated with aqueous phase trapping (or water trapping or blocking as it is often referred to). To understand the concept of aqueous phase trapping, it is essential to differentiate between the concept of initial (often referred to as connate) aqueous phase saturation (Swi) and irreducible aqueous phase saturation (Swirr).Initial aqueous phase saturation is the initial average fractional portion of the pore space which is occupied by water. The value of the initial aqueous phase saturation is controlled by numerous factors, including reservoir geology, depositional history, temperature, wettability, height above free water contact and pore size distribution. The key point to differentiate in this area is that the initial aqueous phase saturation is not necessarily, and often is not equal to the irreducible aqueous phase saturation and can be either higher or lower than the irreducible saturation. It is in the second case, where the Swi is less than Swirr where productivity reductions due to phase trapping can occur.Irreducible aqueous saturation represents that saturation which is forced to exist in the reservoir by capillary mechanics. Once again, the value of the Swirr is determined by parameters such as the reservoir morphology, pore size distribution, pore throat size distribution, wettability, surface roughness, etc. We often obtain estimates of Swirr through the use of air-brine or air-mercury capillary pressure tests, Although these values often provide good approximations to Swirr they may be poor indications of actual Swi. FIGURE 1: Mechanism or aqueous phase trapping. (Available in full paper) Mechanism of Aqueous Phase Trapping Figure 1 provides an illustrative example of a set of relative permeability curves. The diagram is applicable to either an oil or a gas reservoir. Examination of Figure 1 indicates that, if the zone of interest is at some aqueous saturation greater than the irreducible value of 45%, aqueous trapping will not be a severe problem because the reservoir is already initially highly saturated with water and may even be producing free mobile brine.

Nonmobile water quantified in fully saturated porous materials by magnetic resonance relaxation and electrical resistivity measurements

Irreducible fluid saturation in porous media is the fraction of the wetting phase which remains trapped in the solid matrix after displacement with a nonwetting phase. Methods have been proposed to estimate irreducible saturation in fully water-saturated porous samples by means of proton nuclear magnetic resonance relaxation times, but the results are quite varied. Some features are discussed of the trapped fluid and it is shown that the fraction of nonmobile water can be well estimated for a varied suite of clean sandstones, using only measurements on the fully saturated samples. This is done by combining relaxation measurements with those of electrical resistivity factor F. One of several simple correlations is Swi ∝√F/T1s, where T1s is the stretched-exponential relaxation time.

Evaluation of the High-Speed Centrifuge Technique for Determining Capillary Pressure at Low Wetting-Phase Saturations

ABSTRACTRecent experimental studies of the high-speed centrifuge technique have indicated that the method, if properly used, yields results which are in accordance with other methods. These studies have also shown that the concept of an irreducible wetting phase saturation is not valid. In the present paper work is reported in which these previously presented results are extended to both higher and lower capillary pressures. Air/liquid data have been obtained with n-decane as the wetting phase and are compared with new data (as well as with the previously reported data) for the air/brine system. The study involved 242 days of centrifugation and utilized 10 different Berea plug samples and 5 different high permeability plug samples taken from a producing reservoir.The linear relationship between log Pc and log <Sw> (or log Sw, as observed in previous work) was confirmed for Berea samples over the average saturation range from about 30 percent down to 10 percent. For lower average saturations, the saturation data are not sufficiently precise either to (1) demonstrate conclusively the persistence of this relationship, or (2) prove the breakdown of the Hassler boundary condition. For three of the five producing reservoir samples, it was found that two different linear segments characterized the log Pc versus log <Sw> relationship, with a rather sharp transition occurring in the 30 to 40 percent average saturation range.

Interpretation of capillary pressure curves using invasion percolation theory

Invasion percolation was studied on three-dimensional regular lattices of various node numbers. A new model has been developed to obtain the pore-size distribution from capillary pressure measurements. The new model is superior to the conventional percolation model, since it takes into account the physical trapping of the wetting phase. The irreducible wetting phase saturation includes the film of the wall of the pores, the dead-end pore volume, and the main contribution by pores isolated from the outlet of the medium by the nonwetting phase. This has been related to the node number and the sample 3dimensions. Over 100 capillary pressure curves of consolidated media have been collected. Good agreement was obtained between this data set out and our invasion percolation predictions using node numbers of 6–13, as reported by Mishra and Sharma. The pore-throat size distribution function estimated by our new model is broader than from the conventional percolation and the capillary tube models.

LOTUS(tm) Template for Calculating Well Logs

Calculating well logs is a time-consuming process. This template uses input parameters consisting of well name, location county, state, formation name, starting depth, repeat interval, resistivity of shale, and irreducible bulk volume water, which provides heading information for print outs. Required information from basic well logs are porosity, conductivity (optional), formation resistivity, resistivity of the formation water for the zone being calculated, resistivity of the mud filtrate, the porosity cutoff for pay in the zone being calculated, and the saltwater saturation cutoff for the pay zone. These parameters are used to calculate apparent water resistivity, saltwater saturation, bulk volume water, ratio of apparent water resistivity to input water resistivity, irreducible saltwater saturation, resistivity volume of shale, permeability, and a derived porosity value. A print out of the results is available through the lotus print function. Using this template allows maximum control of the input parameters and reduces hand calculation time.

Capillary pressure behavior in pores with curved triangular cross-section: Effect of wettability and pore size distribution

The capillary behavior of two immiscible fluids in pores of angular cross-section differs appreciably from that in pores of circular cross-section. We have extended the work of Mason and Morrow [G. Mason and N.R. Morrow, J. Colloid Interface Sci., 141 (1991) 262; N.R. Morrow, Paper presented at 1st Int. Symp. on Evaluation of Reservoir Wettability and Its Effect on Oil Recovery, Socorro, NM, September 18–21, 1990] to pores whose cross-section is a triangle with curved, rather than straight, sides. Capillary pressure vs saturation curves were obtained for single pores, as well as for bundles of parallel pores with log-normal pore size distributions of varying geometric standard deviation. Wettability, as expressed by the contact angle, was varied also. In many ways, the computed behavior is qualitatively similar to that observed for real porous media, such as petroleum reservoir rocks. That is, there is drainage—imbibition hysteresis, a residual non-wetting-phase saturation upon imbibition, and a reversal in the sign of the capillary pressure upon imbibition in systems that approach neutral wettability. However, because of the lack of network effects, there is no mechanism for a true irreducible wetting-phase saturation. It seems likely that, to capture all aspects of multifluid behavior in real porous media (capillary pressure, relative permeabilities, electrical resistivity, etc.), models of such media should not only be based on networks but must also incorporate pore angularity.

Movement of liquids through powder beds

The movement of hydrophilic fluids into and out of beds of two grades of lactose, microcrystalline cellulose (MCC) and mixtures of the two materials has been assessed by a pressure membrane technique. While water was an appropriate fluid to use with MCC, because of its solvent properties, it proved unsuitable for use with lactose and the binary mixtures with MCC. In these cases, a 50:50% mixture of ethanol and water proved acceptable. The method clearly illustrated the differences in the movement of fluid in the two grades of lactose, which differed in particle size, and between lactose and MCC. These differences could be quantified in terms of limiting saturation pressures, irreducible saturation and ‘apparent’ pore distributions. The MCC introduces absorptive and adsorptive influences into the binary mixtures with lactose which are absent or minimal in the lactose systems. Such effects are undoubtedly involved in ensuring that these binary mixtures can be used to produce spherical granules by extrusion/ spheronisation whereas the process is not successful for lactose alone.

油層浸透性の温度依存性と火攻法の排油機構に関する研究

Temperature dependency of relative permeability of oil and water, irreducible water saturation, and residual oil saturation were measured for sandpack of silica and limestone. And, on the basis of the results obtained, mechanisms of oil displacement in the forward in-situ combustion process were discussed.Results are summarized as follows.(1) The relative permeability of both oil and water decreases with increasing temperature for both silica and limestone sandpacks.(2) The relative permeability of oil for silica sandpack is higher than that for limestone sandpack, though the relative permeability of water for both silica and limestone sandpacks did not show the difference.(3) The irreducible water saturation increases and the residual oil saturation decreases with increasing temperature for both silica and limestone sandpacks. In comparison of residual oil saturation for both sandpacks, that for limestone sandpacks is higher, while the irreducible weter saturation for silica sandpack is higher than that for limestone sandpack.(4) The mobility of oil in the steam bank is higher than that in the virgin zone because of the higher temperature. However, it does not increase drastically as expected from the temperature dependency of oil viscosity because of the counter nature of oil viscosity and relative permeability of oil with temperature.(5) The wide portion of oil bank is not heated up by the heat liberated by combustion, so that the mobility of oil in the oil bank is little improved. However, the movable oil increases due to the nature of decreasing residual oil saturation with increasing temperature because of the temperature increase at the back part of the oil bank. This effect may result in the higher oil recovery.

Secondary Water Recovery by Air Injection: 3. Evaluation of Feasibility

Numerical simulation indicated that secondary water recovery by air injection could be technically feasible in aquifers with thick unsaturated zones, confining top layers, high ratio of horizontal‐to‐vertical permeability, and low intrinsic permeability, and high irreducible air saturations, i.e., clayey textures. Injection at the top of the unsaturated zone, venting the well to the atmosphere immediately after the end of air injection, low‐to‐moderate injection rates and large volumes of injected air seemed to enhance water recovery. Simulated water recovery volumes were significantly lower than the recovery estimates based on water level elevations in observation wells during field tests. The model helped interpret some of the observations made during the field tests of secondary water recovery and helped identify areas of inadequacy in available information needed for representative numerical simulations.