Musaed N.J. Al-Awad

Petrophysical evaluation of low-resistivity sandstone reservoirs with nuclear magnetic resonance log

Abstract The combination of conventional logs, such as density, neutron and resistivity logs, is proven to be very effective in the evaluation of normal reservoirs. For low-resistivity reservoirs, however, an accurate determination of the petrophysical parameters with the conventional log reservoirs is very difficult. This paper presents two cases of low-resistivity reservoirs and low-contrast resistivity reservoirs, where conventional logs fail to determine the petrophysical properties of reservoirs, mainly, low-resistivity and low-contrast resistivity reservoirs. The problems of these reservoirs are that conventional logging interpretation shows high water saturation zones, but water-free hydrocarbon would be produced. In the case of low-resistivity contrast reservoirs, it is very hard to determine water hydrocarbon contact with resistivity logs. Nuclear magnetic resonance (NMR) has only been available as a supplementary tool to provide additional information on the producibility of the reservoir. The main limitations of NMR have been the cost and time of acquiring data. This paper shows that in the case of low-resistivity reservoirs, NMR is a very cost-effective tool and is of help in accurately determining the reservoir rock petrophysical properties. In the analysis of NMR data, several aspects of NMR technique have been used: (1) T1/T2 ratio for fluid identification, (2) the difference between NMR-derived porosity and total porosity to determine the types of clay minerals, (3) NMR relaxation properties to identify fluids composition and rock properties. This paper presents four examples of low-resistivity reservoirs. Analysis of the NMR data of low-resistivity reservoirs has helped identify the producibility of these zones, determine lithology-independent porosity and distinguish between bound and free water. For the case of low-contrast resistivity reservoir, where there was little resistivity contrast between water-bearing formation and oil-bearing formation, NMR has been able to identify the fluid composition of the two formations, as well as the height of the oil column. This was based mainly on the high contrast of NMR relaxation parameters.

A new laminar-to-turbulent transition criterion for yield-pseudoplastic fluids

Abstract A new laminar-to-turbulent transition criterion for predicting the onset of turbulence for yield-pseudoplastic fluids was developed. This criterion is defined as the ratio of the laminar shear stress to the viscous shear stress. The value of the ratio is equal to unity in the viscous sublayer and should be greater than unity in the turbulent zone. The rheological characteristics of the fluids studied were described by a power-law yield-pseudoplastic model. Two equations relating the Metzner and Reed parameters ( n ′ and k ′) to those of yield-pseudoplastic model ( τ y , μ p , and n ) were derived. The developed criterion can also be used to determine the onset of turbulent flow for Bingham and yield-dilatant fluids.

Investigation of Polymer Adsorption on Rock Surface of Highly Saline Reservoirs

Blocking or reducing water production from oil wells is a serious problem in oil industry. Two types of polymers, namely, polyacrylamide (PAA) and polysaccharides (xanthan) have been investigated in this paper. The viscosity of both polymer solutions was first evaluated at different salinities, shear rates and concentrations. Afterwards, the solutions were injected into core samples to examine the adsorption on the rock surface by calculating the resistance factor as well as the residual resistance factor. Also, the effect of the injection rate of the polymer solutions has been studied. The results show that xanthan solution is tolerant of high salinity (20 %), while PAA solution is very sensitive to salt. Both polymer solutions show a pseudoplastic flow as a function of the shear rate. The core sample experiments show that both polymer solutions suffer a reduction in the adsorption rate with salinity increase. However, xanthan shows acceptable values even with a salinity up to 20 % and a temperature of 60 °C. Therefore, xanthan can be recommended to shut off water in high salinity and high temperature reservoirs. It was also found that the lower the injection rate the higher the adsorption on the rock surface.

Nuclear Magnetic Resonance Log Evaluation of Low- Resistivity Sandstone Reservoirs By-Passed by Conventional Logging Analysis

The combination of conventional logs such as density, neutron and resistivity logs is proven to be very effective in the evaluation of normal reservoirs. For low resistivity reservoirs, however, an accurate determination of the petrophysical parameters with the conventional log reservoirs is very difficult. This paper presents two cases of low resistivity reservoirs and low contrast resistivity reservoirs where conventional logs fail to determine the petrophysical properties of reservoirs, mainly, low resistivity and low contrast resistivity reservoirs. This paper shows that in the case of low resistivity reservoirs NMR is very costeffective tool and helps to accurately determine the reservoir rock petrophysical properties. In the analysis of NMR data, several aspects of NMR technique have been used; 1) T1/T2 ratio for fluid identification, 2) the difference between NMR derived porosity and total porosity to determine the types of clay minerals, 3) NMR relaxation properties to identify fluids nature and rock properties. This paper presents four examples of low resistivity reservoirs. Analysis of NMR data of low resistivity reservoirs has helped to identify the productivity of these zones, to determine lithology independent porosity and to distinguish between bound and free water. For the case of low contrast resistivity reservoir where there was little resistivity contrast between water bearing formation and oil bearing formation, NMR has been able to identify the fluid nature of the two formations and then the height of the oil column. This was based mainly on high contrast of NMR relaxation

Petrophysical Evaluation of Low Resistivity Sandstone Reservoirs

There are many reasons for low resistivity pay zones phenomenon. It is of crucial importance to know the origin of this phenomenon. The problem with these zones is that the resistivity data interpretation indicates high water saturation, but oil or even dry oil will be produced. This paper discuss the different reasons sandstone reservoir can have low resistivity. Clean oil bearing sandstone has high resistivity, but when this rock contains shale, or heavy minerals such as pyrite, the resistivity can become low. This paper deals with the case of shaly sand formation as a low resistivity pay zone. Different shaly sand models will be applied. It has been found that the modified total shale sand model gives good results. Field example is presented to show the results of different models. INTRODUCTION The reasons for low resistivity phenomenon are classified mainly into two groups. The first consists of reservoirs where the actual water saturation can be high, but water free hydrocarbons are produced. The mechanism responsible for the high water saturation is usually described as being caused by microporosity. The second group consists of reservoirs where the calculated water saturation is higher than the true water saturation. The mechanism responsible for the high water saturation is described as being caused by the presence of conductive minerals such as clay minerals and pyrite in a clean reservoir rock. The resistivity data must be corrected for the effect of these conductive minerals to reduce the calculated water saturation to the more reasonable levels associated with water free hydrocarbon production. Most formations logged for potential oil or gas production consist of rocks which without fluids would not conduct an electrical current. There are two types of rock conductivity: a) Electrolytic conductivity which is a property of for instance water containing dissolved salts and b) Electronic conductivity which is a property of solids such as graphite and metal sulfides such as pyrite. This paper deals with the case of low resistivity pay zone incurred by the occurrence of clay minerals and Pyrite. Field example will be provided to show how to deal with this problem. SHALE SAND MODELS AND WATER SATURATION In the last years, a considerable number of shaly sand models relating resistivity to water saturation have been proposed, for details of algorithms for shale sand models see some references ( Patchett and Rausch, 1967; Tixier et al , 1968; Fertl and Hammack, 1971; Zemanek, 1989; Aguilera, 1990; Hamada, 1996 ). All these models are composed of two terms, a clean sand term described generally by Archies equation and a shale term. The shale term could be fairly simple or very complicated. (Simandoux , 1963 ; Poupon et al, 1954 ) have shown that in some cases it is possible to use the following total shale model equation to calculate water saturation,independently of the distribution of shale: Sw = ( aRw / φ Rt + (aRw Vsh / 2 φRsh ) ) (aRw Vsh / 2 φRsh) (1) The above equation has been widely accepted and applied in many areas including Nigeria, Argentina, Egypt’, USA, Saudi Arabia and Libya. One limitation is that the porosity exponent is taken as 2 and the value of aRw has to be accurately identified. To overcome this limitation, we introduce both cementation exponent m and saturation exponent n as variables and rewrite Equation 1. The modified total shale equation will be used in the analysis of the field example. FIELD EXAMPLE This example is taken from a shaly sandstone formation with low resistivity. This well has been discussed and evaluated previously (Aguilera 1990) using one of the shale sand modeling technique developed by Schlumberger. Figure 1 shows SP, resistivity, neutron and density logs. In this example, the total shale model ( Equation 1 ) was used after modification for the evaluation. This model was modified to include a, n and m constants. The Humble formula constants were selected which were a = 0.62 and m = 2.15. Equation 1 was modified to include the shale term Atsh to the following form: Rt / Atsh = a Rw φ -m Sw -n (2) The total shale group Atsh is given by Atsh = 1 + φRt / a Rw (2Btsh 2Btsh (aRw / φRt + Btsh)) ( 3)

Two New Chemical Components for Sand Consolidation Techniques

Sand production problems are encountered throughout the world and recently detected in Saudi Arabian oil fields. Therefore an increased emphasis is being placed on proper initial well completion as the value of non-renewable oil reserves increases and cost of remedial work skyrockets. Sand control by consolidation involves the process of injecting chemicals into the naturally unconsolidated formation to provide in situ grain-to-grain cementation. Techniques for accomplishing this successfully are perhaps the most sophisticated ones undertaken in completion work. Many methods have been suggested to consolidate the wall of the wellbore for few inches or feet around the hole. These methods are either expensive or temporarily. This paper introduces two new cheep chemical components to be used to consolidate friable sand formation at temperature up to 300oC. The paper introduces these two components, discusses the physical and petrochemical properties of the consolidated sand and the factors affecting this consolidation and highlight the laboratory process and field application of these two components. Introduction Sand production problems are experienced in many oil and gas productive formations [1]. They are most significant in unconsolidated sandstone reservoirs. Sand influx into the wellbore may lead to various problems such as erosion of valves and pipelines, plugging the production liner and sand accumulation in the separators. Cleaning and repair works related to sand production plus loss of revenue due to production rate restriction amounts to great costs incurred by the industry every year. Furthermore, undetected erosion of production equipment may pose a major safety hazard in case of high-pressure gas wells. Therefore, sand control has attracted much research effort for more than six decades [2]. Sand production is explained in several ways. The most convincing theory attributes sand production to friction and resultant pressure drop as well fluid passes through the small pores of the sand body. If the cementing materials are not strong enough and that the pressure drop is high, the individual sand grain is displaced and carried into the wellboe. Another plausible explanation considers the fact that the formation compaction as the bore pressure decreases, and the variations of the load, tends to shift sand grains and shear the cementing material. Another strong explanation highlights the chemical difference between the water initially present when the sand grains were first deposited and that water contained in the aquifer. Water production can actually dissolve a part of the cementing material between sand grains. SPE 68225 Two New Chemical Components for Sand Consolidation Techniques Abdel-Alim H. El-Sayed*, Musaed N. Al-Awad & Emad Al-Homadhi, King Saud University, Riyadh, Saudi Arabia,

Determining Petrophysical Properties of Low Resistivity Reservoirs Using Nuclear Magnetic Resonance Logs

The combination of conventional logs such as density, neutron and resistivity logs is proven to be very effective in the evaluation of normal reservoirs. An accurate determination of the petrophysical parameters with the conventional logs for low resistivity reservoirs is very difficult. This paper presents the two cases of low resistivity reservoirs and low contrast resistivity reservoirs. The problem of these reservoirs is that conventional logging interpretation shows water zones, but water free hydrocarbon would be produced. In the case of low resistivity contrast reservoirs it is very hard to determine water hydrocarbon contact with resistivity logs. Nuclear magnetic resonance (NMR) has been only available as a supplement tool, to provide additional information on the producibility of the reservoir. The main limitation of NMR, however, has been the cost and time of acquiring data. This paper shows that in the case of low resistivity reservoirs NMR is very cost-effective tool and helps to accurately determine the reservoir rock petrophysical properties. In the analysis of NMR data, several aspects of NMR technique have been used; 1) T1/T2 ratio for fluid identification, 2) the difference between NMR derived porosity and total porosity to determine the types of clay minerals, 3) NMR relaxation properties to identify fluids nature and rock properties This paper presents four examples of low resistivity reservoirs. Analysis ofNMR data oflow resistivity reservoirs has helped to identify the producibility of these zones, to determine lithology independent porosity and to distinguish between bound and free water. For the case of low contrast resistivity reservoir where there was little resistivity contrast between water bearing formation and oil bearing formation, NMR has been able to identify the fluid nature of the two formations and then accurately the height of the oil column. This was based mainly on high contrast ofNMR relaxation parameters.

Effect of triaxial in situ stresses and heterogeneities on absolute permeability of laminated rocks

This work aims at improving our understanding of the effect of triaxial stress on absolute permeability of homogeneous and heterogeneous rocks. Measurements of absolute permeabilities of homogenous and heterogeneous laminated rock samples under hydrostatic and different laboratory-simulated triaxial in situ stress loadings were prepared. Experiments were conducted using homogenous, standard Berea and heterogeneous laminated sandstone cores (with lamination parallel to the flow direction). The effect of in situ stress variation on absolute permeability was investigated. Absolute permeability of homogenous sandstones sample decreased in a uniform manner as the axial load increased below the ultimate shear strength. As axial stress exceeds the radial confining stress, slight increase attributed to microcracks opening was noticed. For laminated sandstones, the scenario is quite different. Permeability shows exactly a similar trend of permeability drop as axial load increases due to inferred matrix compaction. At certain axial load, the permeability starts to increase due to inferred dilatancy of microcracks at the lamina faces and then drops again as axial load increases due to lamina compaction. If the axial load is further increased, pore collapses and grain-to-grain cementing breaks down into pore space followed by microcracks development predominantly parallel to the axial load leading to permeability enhancement. It has been concluded that absolute permeability changes due to lamination opening and closure as a result of loading magnitude and orientation. It is also concluded that permeability of the formation rock is affected by heterogeneity depending on the direction of lamination as well as the state of the stresses applied and loading type.

Utilization of Shredded Waste Car Tyres as a Fracture Seal Material (FSM) in Oil and Gas Drilling Operations

The drilling operation cost represents 25% of the total oilfield exploitation cost. Drilling fluids represent 15 to 18% of the total cost of well petroleum drilling operations. The main drilling fluids problem is the loss into fractures and vugs. Mitigation of severe lost circulation is a main challenge while drilling in fractured formations where conventional lost circulation materials (LCM) will not cure these losses. Therefore, specialized fracture seal material (FSM) is required when drilling fractured formations. In this study, a promising FSM made from shredded waste car tyres was tested at laboratory for its ability to seal artificially fractured cores under High Temperature High Pressure (HT-HP) conditions similar to wellbore conditions. For this purpose, the conventional 500 ml HT-HP filtration cell was modified to accommodate a fractured core plug of length and diameter equal to 38.1 mm (1.5 inches) instead of the ceramic disc. Moreover, the cell outlet channel located below the fractured plug was increased from 1.0 mm diameter to 5.0 mm to easily allow the passage of the FSM in none effective fracture seal tests. Using the modified HT-HP filtration cell shredded waste car tyres proved its ability to perfectly seal the artificially made fracture in the test core samples at overbalance pressures up to 900 psi and temperatures up to 80°C. The optimum mud composition was fresh water, 7% by weight bentonite, 7% shredded waste car tyres (a mixture ranging between 2.3 mm and less than 0.45 mm granule sizes) in weight bases. In addition to its great ability to seal fractured formation, the shredded waste car tyres material is cheap and locally available in commercial quantities. Additionally, the utilization of waste car tyres in drilling operations and other industrial applications can protect the environment from many hazards.

Estimating the Amount of Free Sand in the Yielded Zone Around Vertical and Horizontal Oil Wells

Predicting sand production accurately is a difficult task; many techniques have been previously investigated such as production history, mechanical property analysis using electrical log data, laboratory testing and computer modeling. In this study, the mechanism of sand production problem in an oil reservoir producing medium oil (30 o API) from a weak sandstone formation was investigated. An analytical model is elaborated based on linear-poroelastic solution of stress state around circular openings as well as Mohr-Coulomb failure criterion and Darcy’s equations for fluid flow through porous media in vertical and horizontal wellbores. In this study, a new important factor necessary for the estimation of the mount of free sand generated between sheared planes caused by the fluid drawdown was introduced. This factor is called sand production capability factor. Sand capability factor was evaluated experimentally for the studied reservoir. Observations of sand production in the studied oil reservoir were utilized to tune and verify the model used in this study. For open-hole completion, it was found that; free sand ready to move into the wellbore is inversely proportional to radial distance. Furthermore, free sand ready for production from the yielded zone around vertical wells is higher than that in the case of horizontal wells. The predicted free sand in all studied cases is in accordance with field observations. Selection of borehole and perforation orientation (in case of perforated casing completion) with respect to the maximum horizontal principal in-situ stress has a great effect in reducing the potential free sand amounts ready to move into the wellbore along with the producing reservoir fluids. Horizontal wellbores oriented at 45 o produce minimum sand compared with other horizontal orientations for the studied reservoir. Similar effect is found for perforations phased at zero angular position.