Mineralogy and Mineral Sensitivity of Petroleum-Bearing Formations
Among others, Ohen and Civan 1993 point out that fines migration and clay swelling are the primary reasons for formation damage measured as permeability impairment. Poorly lithified and tightly packed formations having large quantities of authigenic, pore filling clays sensitive to aqueous solutions, such as kaolinite, illite, smectite, chlorite, and mixed-layer clay minerals, are especially susceptible to formation damage Amaefule et al., 1988. Formation damage also occurs as a result of the invasion of drilling mud, cement, and other debris during production, hydraulic fracturing, and workover operations Amaefule et al., 1988. This article describes the mineral content and sensitivity of typical sedimentary formations, and the relevant formation damage mechanisms involving clay alteration and migration. Analytical models for interpretation and correlation of the effects of clay swelling on the permeability and porosity of clayey porous rocks are presented Civan, 1999. The parameters of the models, including the swelling rate constants, and terminal porosity and permeability that will be attained at saturation, are determined by correlating the experimental data with these models. The swelling of clayey rocks is essentially controlled by absorption of water by a water-exposed surface hindered diffusion process and the swelling-dependent properties of clayey rocks vary proportionally with their values relative to their saturation limits and the water absorption rate. These models lead to proper means of correlating and representing clayey rock properties.
Origin of Petroleum-Bearing Formations
As described by Sahimi 1995, sedimentary porous formations are formed through two primary phenomena:
1. deposition of sediments, followed by
2. various compaction and alteration processes.
Sahimi 1995 states that the sediments in subsurface reservoirs have undergone four types of diagenetic processes under the prevailing in-situ stress, thermal, and flow conditions over a very long period of geological times:
1. mechanical deformation of grains,
2. solution of grain minerals,
3. alteration of grains, and
4. precipitation of pore-filling minerals, clays, cements, and other materials. These processes are inherent in determining the characteristics and formation damage potential of petroleum bearing formations.
Constituents of Sedimentary Rocks
Many investigators, including Neasham 1977, Amaefule et al. 1988, Macini 1990, and Ezzat 1990, present detailed descriptions of the various constituents of oil and gas bearing rocks. Based on these studies, the constituents of the subsurface formations can be classified in two broad categories:
1. indigenous and
2. extraneous or foreign materials.
There are two groups of indigenous materials:
1. detrital materials, which originate during the formation of rocks and have restricted formation damage potential, because they exist as tightly packed and blended minerals within the rock matrix; and
2. diagenetic or authigenic materials, which are formed by various rock-fluid interactions in an existing pack of sediments, and located inside the pore space as loosely attached pore-filling, pore-lining, and pore-bridging deposits, and have greater formation damage potential because of their direct exposure to the pore fluids.
Extraneous materials are externally introduced through the wells completed in petroleum reservoirs, during drilling and workover operations and improved recovery processes applied for reservoir exploitation.
Composition of Petroleum-Bearing Formations
The studies of the composition of the subsurface formations by many, including Bucke and Mankin 1971 and Ezzat 1990, have revealed that these formations basically contain:
1. various mineral oxides such as SiO2, A12O3, FeO, Fe2O3, MgO, K2O, CaO, P2O5, MnO, TiO2, Cl, Na2O, which are detrital and form the porous matrix, and
2. various swelling and nonswelling clays, some of which are detrital, and the others are authigenic clays. The detrital clays form the skeleton of the porous matrix and are of interest from the point of mechanical formation damage. The authigenic clays are loosely attached to pore surface and of interest from the point of chemical and Physico-chemical formation damage.
However, the near-wellbore formation may also contain other substances, such as mud, cement, and debris, which may be introduced during drilling, completion, and workover operations, as depicted by Mancini 1991.
"Clay" is a generic term, referring to various types of crystalline minerals described as hydrous aluminum silicates. Clay minerals occupy a large fraction of sedimentary formations Weaver and Pollard, 1973. Clay minerals are extremely small, platy-shaped materials that may be present in sedimentary rocks as packs of crystals Grim, 1942; Hughes, 1951. The maximum dimension of a typical clay particle is less than 0.005 mm Hughes, 1951. The clay minerals can be classified into three main groups Grim, 1942, 1953; Hughes, 1951:
1. Kaolinite group,
2. Smectite or Montmorillonite) group, and
3. Illite group. In addition, there are mixedlayer clay minerals formed from several of these three basic groups Weaver and Pollard, 1973.
Mineral Sensitivity of Sedimentary Formations
Among other factors, the interactions of the clay minerals with aqueous solutions is the primary culprit for the damage of petroleum-bearing formations. Amaefule et al. 1988 state that rock-fluid interactions in sedimentary formations can be classified in two groups:
1. chemical reactions resulting from the contact of rock minerals with incompatible fluids, and
2. physical processes caused by excessive flow rates and pressure gradients.
Amaefule et al. (1988) point out that there are five primary factors affecting the mineralogical sensitivity of sedimentary formations:
1. Mineralogy and chemical composition determine the
A. dissolution of minerals,
B. swelling of minerals, and
C. precipitation of new minerals.
2. Mineral abundance prevails the quantity of sensitive minerals.
3. Mineral size plays an important role, because
A. mineral sensitivity is proportional to the surface area of minerals, and
B. mineral size determines the surface area to volume ratio of particles.
4. Mineral morphology is important, because
A. mineral morphology determines the grain shape, and therefore the surface area to volume ratio, and
B. minerals with platy, foliated, acicular, filiform, or bladed shapes, such as clay minerals, have high surface area to volume ratio.
5. Location of minerals is important from the point of their role in formation damage. The authigenic minerals are especially susceptible to alteration because they are present in the pore space as pore-lining, pore-filling, and pore-bridging deposits and they can be exposed directly to the fluids injected into the near-wellbore formation.
Mungan 1989 states that clay damage depends on 1. the type and the amount of the exchangeable cations, such as K+, Na+, Ca2+, and
2. the layered structure existing in the clay minerals. Mungan 1989 describes the properties and damage processes of the three clay groups as following:
1. Kaolinite has a two-layer structure K+ exchange cation, and a small base exchange capacity, and is basically a nonswelling clay but will easily disperse and move.
2. Montmorillonite has a three-layer structure, a large base exchange capacity of 90 to 150 meq/lOOg and will readily adsorb Na+, all leading to a high degree of swelling and dispersion.
3. Illites are interlayered.Therefore, illites combine the worst characteristics of the dispersible and the swellable clays. The illites are most difficult to stabilize.
Sodium-montmorillonite swells more than calcium-montmorillonite because the calcium cation is strongly adsorbed compared to the sodium cations Rogers, 1963. Therefore, when the clays are hydrated in aqueous media, calcium-montmorillonite platelets remain practically intact, close to each other, while the sodium-montmorillonite aggregates readily swells and the platelets separate widely. Therefore, water can easily invade the gaps between the platelets and form thicker water envelopes around the sodium-montmorillonite platelets than the calcium-montmorillonite platelets Chilangarian and Vorabutr, 1981
Clay damage can be prevented by maintaining high concentrations of K+ cation in aqueous solutions. At high concentrations of K+ cation, clay platelets remain intact, because the small size K+ cation can penetrate the interlayers of the clay easily and hold the clay platelets together Mondshine, 1973 and Chiligarian and Vorabutr, 1981.
Many investigators, including Mungan 1965, Reed 1977, Khilar and Fogler 1983, and Kia et al. 1987, have determined that some degree of permeability impairment occurs in clay containing cores when aqueous solutions are flown through them. This phenomenon is referred to as the "water sensitivity." Reed (1977) observed that young sediments are mostly friable micaceous sands and proposed a mechanism for damage.
To justify his theory, he also conducted laboratory core tests by flowing various aqueous solutions through cores extracted from micaceous sand formations.
Mohan and Fogler 1997 explain that there are three processes leading to permeability reduction in clayey sedimentary formations:
1. Under favorable colloidal conditions, non-swelling clays, such as kaolinites and illites, can be released from the pore surface and then these particles migrate with the fluid flowing through porous formation Mohan and Fogler, 1997.
2. Whereas swelling clays, such as smectites and mixed-layer clays, first expand under favorable ionic conditions, and then disintegrate and migrate Mohan and Fogler, 1997.
3. Also, fines attached to swelling clays can be dislodged and liberated during clay swelling, the phenomenon of which is referred to as fines generation by discontinuous jumps or microquakes by Mohan and Fogler 1997.
Consequently, formation damage occurs in two ways:
1. the permeability of porous formation decreases by reduction of porosity by clay swelling Civan and Knapp, 1987; Civan et al., 1989; and Mohan and Fogler, 1997; and
2. the particles entrained by the flowing fluid are carried towards the pore throats and captured by a jamming process.
Thus, the permeability decreases by plugging of pore throats Sharma and Yorstos, 1983; Wojtanowicz et al., 1987, 1988; Mohan and Fogler, 1997. Khilar and Fogler (1983) have demonstrated by the flow of aqueous solutions through Berea sandstone cores that there is a "critical salt concentration CSC" of the aqueous solution below which colloidally induced mobilization of clay particles is initiated and the permeability of the core gradually decreases. This is a result of the expulsion of kaolinite particles from the pore surface due to the increase of the double-layer repulsion at low salt concentration Mohan and Fogler, 1997.
Mechanism of Clay Swelling
A structural model of swelling clays having exchangeable cations, denoted by Mz+, is shown by Zhou et al. 1996, 1997 Zhou et al 1996 states: "The structure layers are always deficient in positive charges due to cation substitution, and interlayer cations are required to balance the negative layer charge. Interlayer cations are exchangeable and the exchange is reversible for simple cations. The distance between two structure layers, i.e. 001 d-spacing, is dependent on the nature type of the exchangeable cation, composition of the solution, and the clay composition. Clay swelling is a direct result of the d-spacing increase and volume expansion when the exchangeable cations are hydrated in aqueous solution."
As stated by Zhou 1995, "clay swelling is a result of the increase in interlayer spacing in clay particles." Clay swelling occurs when the clay is exposed to aqueous solutions having a brine concentration below the critical salt concentration Khilar and Fogler, 1983. Therefore, Zhou 1995 concludes that "clay swelling is controlled primarily by the composition of aqueous solutions with which the clay comes into contact." Norrish 1954 have demonstrated by experiments that clay swelling occurs by crystalline and osmotic swelling processes. Zhou 1995 explains that:
1 crystalline swelling occurs when the clays are exposed to concentrated brine or aqueous solutions containing large quantities of divalent or multivalent cations. It is caused by the formation of molecular water layers on the surface of clay minerals. This leads to less swelling and less damage; and
2 osmotic swelling occurs when the clays are exposed to dilute solutions or solutions containing large quantities of Na+ cations. It is caused by the formation of an electric double layer on the surface of clay minerals. It leads to more swelling and more damage. These phenomena create repulsive forces to separate the clay flakes from each other.
Mohan and Fogler 1997 conclude that crystalline swelling occurs at high concentrations below the critical salt concentration and osmotic swelling occurs at low concentrations above the critical salt concentrations. Mohan and Fogler 1997 measured the interplanar spacing as an indication of swelling of montmorillonite in various salt solutions.
These charts indicate the cation concentrations of aqueous solutions that will cause crystalline or osmotic swelling. Consequently the cation compositions that will lead to formation damage can be identified readily in the region of the osmotic swelling.
Models for Clay SwellingIn this section, the analytical models by Civan 1999 are presented for interpretation and correlation of measurements of swelling-dependent
cause of water transfer through clayey porous formations. But, transfer rates tend to increase with pressure application. Ballard et al. 1994 observed that, beyond a certain threshold pressure, water and ions move at the same speed. This is because transfer by advection dominates and diffusion by concentration gradients becomes negligible.
The Civan and Knapp 1987 and Civan et al. 1989 models for variation of porosity and permeability by swelling assume that the external surface of the swelling clay is in direct contact with water at all times and therefore they used a Dirichlet boundary condition in the analytic solution of the models. Civan 1999 developed improved models by considering a water-exposed-surface-hindered-diffusion process and used a Neumann boundary condition in the analytical solution of the models. By means of a variety of experimental data reported in the literature, Civan 1999 demonstrated and verified that this boundary condition leads to improved analytic models which correlate the experimental data better as closely as the quality of the data permits. He has also shown that the various phenomenological parameters, such as the rate constants and the terminal porosity and permeability values that will be attained at water saturation, can be conveniently determined by fitting these models to experimental data. Civan 1999 pointed out that the laboratory swelling tests are generally carried out using aqueous solutions of prescribed concentrations. Whereas, the composition of aqueous solutions in actual reservoir formations may vary, but this effect can readily be taken into account by incorporating a time-dependent clay surface boundary condition by applying Duhamel's theorem. As a result, the effect of variable aqueous solution concentration can be adequately incorporated into the simulation of formation damage by clay swelling.
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