This paper presents the results of an experimental study performed to investigate the effect of reduced interfacial tension (IFT) on oil recovery by spontaneous imbibition and gravity segregation. The results of the static imbibition experiments, in which a matrix block was surrounded by the imbibing fluid, showed that a minimum Bond number (NB-1) was required to increase the final oil recovery. However, the reduction of the IFT may increase or decrease the rate of imbibition substantially depending on the relative contribution of capillary and gravity forces. The results of the experiments performed on three ranges of permeabilities with four different surfactant solutions (different IFT values) showed that imbibition can occur in three different regimes: capillary dominated, gravity dominated, and a regime in which both forces affect the imbibition process. In order to examine the effect of IFT on oil recovery on the same core, core wettability has to be restored after each test. Several cleaning procedures were investigated to reproduce original core wettability. As an outgrowth of this research, a core cleaning technique for restoring core wettability has been suggested. Firing the core after toluene cleaning for seven hours at 950 F was found to be an effective procedure to restore core wettability after solvent extraction. The same procedure also proved effective for restoring the original core wettability after surfactant solution imbibition tests. Imbibition has been recognized as an important factor in recovering oil from water-wet fractured reservoirs subjected to waterflood or water drive. Water imbibition is the process by which water flows into porous media by capillary forces. These forces allow the water to flow from the fractures into the water-wet, oil-saturated matrix blocks, which causes the oil to be displaced from the matrix into the fracture. Unfortunately, water imbibition is inefficient in recovering oil, often leaving more than half of the original oil-in-place unrecovered. A static imbibition by surfactant solution imbibition process recovers residual oil by reducing the surface forces between the oil and the imbibing fluid. On the other hand, by reducing the interfacial tension, the capillary forces that cause imbibition are reduced. Objective: The objective of this static imbibition work is to investigate the effect of reducing IFT in different oil/water/rock systems on final oil recovery. Methods of Investigation: Sets of experiments were performed by using a specially designed static imbibition apparatus. Each experiment consisted of exposing an oil-saturated Berea sandstone core to water imbibition (base case), cleaning and saturating the core with the same oil, and exposing it to surfactant solution imbibition. Oil recovery by water and surfactant solutions imbibition from each core are compared. P. 107

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A joint-industry project (JIP) from 2005 through 2009 involved acquiring, analyzing, and documenting more than 100 subsalt wells in the Gulf of Mexico (GOM), from shelf to deepwater. The result of this unique study culminated in the compilation of a large and comprehensive database of pertinent petrophysical, geophysical, and drilling data. This database was used to develop a set of suggested best practices for pore-pressure estimation below and around salt bodies. An analysis of the wells in the subsalt JIP indicated that the majority (64%) of the wells studied demonstrated evidence of a subsalt gouge or rubble zone. Sufficient data for this analysis was obtained from 87 JIP wells. Statistical analysis of this large database allowed a wide range of correlations to be established for the subsalt rubble-zone occurrence with salt body structural and geological features. An early diagnostic of rubble zones using logging while drilling (LWD) tools can trigger certain drilling practices to be employed. This paper presents the diagnostic criteria for a subsalt rubble zone and several other confirmatory criteria. The diagnostic identification for a subsalt rubble zone is small-scale discordance between the gamma ray and the resistivity log in the interval immediately below the salt. The relationship between rubble-zone occurrence and salinity changes is discussed.Drilling subsalt rubble zones can be challenging. This paper documents current industry methods for drilling subsalt wells based on the analysis of multiple sources of data. In addition to the subsalt database, data was compiled from an industry questionnaire designed to capture current practices for drilling subsalt wells, a survey and reviews on subsalt drilling-related published literature, and personal interviews with industry professionals. This paper focuses on the following key subsalt (with and without a rubble zone) drilling issues of interest to wellconstruction professionals: the drilling window, casing setting, loss of circulation, wellbore stability, mud-weight scenarios, and leakoff tests. Highlights of each of the aforementioned drilling issues are presented along with well examples.

Foam flow in porous media has been a subject of extensive investigation for the last 30 years because of its application in enhanced oil recovery (BOR) and stimulation operations. In acidizing, foam is used to facilitate the diversion of acid into the low-permeability strata when multiple layers of contrasting permeabilities are present. Very little has been done to investigate the flow behavior of foam in low-permeability rocks (1–10 md), mainly because of equipment limitations caused by the very high pressure gradients encountered when foam is injected. This paper discusses the results of several single-core, constant-quality steady-state foam flow experiments, using a 9-md fired Berea Sandstone core (11" -long, 1-1/2 diameter) and a 3500-psi-rated coreflooding unit. Factors, such as surfactant type, foam quality, liquid and gas velocities are varied to investigate their effect on foam mobility, mobility reduction factors and pressure gradients. To simulate the post-foam acid stage used in acidizing, brine is injected after the foam in each experiment and the residual permeability and foams stability were observed. The foam qualities varied from 35% to 90%, and the injection rates varies from 5 to 25 ft/day. Significant reductions in mobilities are observed for all the cases during steady-state foam injection. A new parameter called the persistency index is proposed to quantify foam stability during the post-foam acid stage, which may prove to be critical to predicting the foam diversion behavior. Also, more consistent results were obtained when dimensionless parameters, such as mobility reduction factors rather than mobilities were compared. Finally, low-permeability results were compared with those for the higher-permeability to identify uniqueness of foam generation in low-permeability formations. Introduction Acidizing is one of the most commonly-used stimulation techniques to treat the damaged and the low-permeability formations in order to improve hydrocarbon recovery. However, when multiple layers of different permeabilities are present, acid flows more into the high-permeability layers than the low-permeability layer. Foam is considered to be the ideal fluid of choice because it is clean and it can be easily removed after treatment. Theoretical and experimental works on foam flow through porous media indicate that foam performance is influenced by factors such as absolute permeability, permeability contrast, foam quality, flow rate, surfactant type, surfactant concentration, and others. Identifying the effects of each of these factors is critical to a successful foam diversion treatment. Effect of Permeability Bernard and Holm first identified the effectiveness of foam in reducing gas mobility with increase in the absolute permeability and its effectiveness to control channeling due to permeability variation. Raza reported that foam can improve sweep efficiency of fluid injection processes because of its flow impeding characteristics and its inherent capability of providing greater flow impedance in high permeability porous media than in low permeability media. He also reported that in heterogeneous reservoirs, the generation of foam in the high permeability streaks will not only impede the entry of fluids in these streaks but will also divert the flow into the tighter streaks. Hirasaki postulated that permeability can affect mobility through its effects on gas mobility and on the foam texture. This observation was analogous to what was observed in smooth capillaries. Khatib et al. speculated that the gas mobility could be an increasing or decreasing function of the permeability as different mechanisms dominated the foam stability and thus the foam texture. P. 261^