Determination of the Formation Damage Potential by Laboratory Testing
Haggerty and Seyler (1997) conducted an extensive laboratory investigation of formation damage by mud cleanout acids and injection waters in Aux Vases sandstone reservoirs. They determined that typical Aux Vases reservoir formation is "a poorly cemented, soft, friable, fine-grained sandstone with pores lined with diagenetic clay minerals. The diagenetic clay mineral suite in Aux Vases reservoirs is a closely intergrown mixture of mixed-layered illite/smetite, chlorite, and illite. No kaolinite was found in the Aux Vases reservoir rocks sampled." They recommended that injection waters should be as saline as the formation brines and a properly formulated mud cleanout acid should be used to reduce formation damage.
Seyler (1998) conducted an extensive petrographical analyses of the core samples obtained from Aux Vases reservoirs. Seyler (1998) examined over 150 thin sections.
The petrographical analyses conducted included:
1. Standard optical microscopy using thin sections. For this purpose, they stained the thin sections with potassium ferricyanide and alizarine red to distinguish and detect the carbonate phases. Petro
graphical characteristics and attributes, such as grain composition and size, cementing agents, porosity types, and reservoir quality were determined.
2. X-ray diffraction analysis (XRD). The XRD analyses determined the type and semi-quantitative composition of the minerals present in the samples.
3. Scanning electron microscopy (SEM) with energy dispersive x-ray microbeam (EDX) analysis, referred to as the SEM/EDX analysis.
For this purpose, the samples were sputter-coated with gold and palladium. The SEM analyses identified the pore-lining minerals and the EDX analysis determined the elemental composition.
The analyses of the individual core samples are presented by Haggerty and Seyler (1997). They concluded that Aux Vases formation core samples contained 65-90% quartz, 3-15% feldspar, 0-15% calcite and 2-7% clay minerals.
Haggerty and Seyler (1997) determined that calcite is the primary porefilling mineral. They described the observed three types of pore filling calcite as: "In relative order of abundance, they are
1 patchy cement filling intergranular porosity;
2 framework grains such as marine fossil fragments, ooids, and pelloids; and
3 minute, late-stage euhedral crystals on diagenetic clay minerals that coat framework grains and line pores.
" Haggerty and Seyler (1997) describe the pore-lining minerals to be, "in descending order of abundance, dominantly diagenetic clay minerals, calcite, partially dissolved feldspars, solid hydrocarbons, anatase, barium-rich celestite, and traces of dolomite." Haggerty and Seyler (1997) observed that "Pores in Aux Vases sandstone reservoirs are lined with, and may be bridged by, diagenetic clay minerals that consist of an intimately intergrown mixture of mixed-layered illite/smectite, chlorite, and illite. Although clay minerals constitute only 2-7% of the bulk mineral content, SEM analysis indicates that clay minerals coat more than 95% of pore surfaces.
Therefore, an understanding of the composition and response of these diagenetic clay minerals to injected fluids is of utmost importance when selecting drilling muds and stimulation methods." "Chlorite identified by XRD and SEM/EDX analyses in Aux Vases samples is typically not iron-rich, but contains approximately equal amounts of iron and magnesium." "Reservoirs containing chlorite rich in iron may be more susceptible to formation damage than those containing other varieties of chlorite because they may form insoluble iron oxides or iron hydroxides." Haggerty and Seyler (1997) report that the mixed-layered illite/montmorillinite (smectite) varieties are the only water sensitive, expandable clay minerals they found in the Aux Vases core samples.
Focus and Design of Experimental Studies
The primary objective of the studies by Haggerty and Seyler (1997) is the Experimental Investigation of Formation Damage by
1 mud cleanout acids and
2 injection waters in Aux Vases sandstone reservoirs.
Haggerty and Seyler (1997) describe that: "Development of sandstone reservoirs in the Illinois Basin typically includes these steps:
1. Drilling with freshwater mud;
2. Perforating the potential reservoir zone, if casing is used, or open hole completions with casing cemented above the producing zone;
3. Preflushing with 15% hydrochloric acid (HCL) or mud cleanout acid (MCA) to remove drilling mud;
4. Cleaning out perforations or the well bore with MCA;
5. Initial swabbing to retrieve stimulation fluids and induce oil flow toward the wellbore;
6. Hydraulic fracturing using a freshwater gelled pad and sand propant;
7. Final swabbing during the production test.
In an effort to simulate the field practice in the laboratory investigations, Haggerty and Seyler (1997) state that: "The experiments focused on five objectives:
1. Determine how MCA containing 15% HCL with additives, typically used during completion and/or stimulation, affects pore-lining minerals and the permeability of Aux Vases reservoir rocks by conducting dynamic, constant rate injection coreflood experiments;
2. Investigate how 15% HCL and MCA affects crude oil from Aux Vases reservoirs by conducting compatibility experiments;
3. Examine how exposure to fluids of various salinities affects permeability in samples of Aux Vases reservoirs by conducting coreflood experiments;
4. Investigate the effects of long-term contact of 15% HCL and MCA with pore-lining minerals in reservoir samples by conducting static soak experiments;
5. Compare XRD analyses of the bulk mineralogy and SEM/EDX analyses of pore-lining minerals with flood results to identify minerals that would be most affected by fluids commonly used during drilling, completion, and stimulation of Aux Vases reservoirs.
Therefore, Haggerty and Seyler (1997) carried out five sets of bench experiments, with the specific objectives described in this article. The direct contact experiments have been conducted to determine the effect of the acids on the physical properties of crude oil. In the coreflood tests, they continuously injected excessive amounts (25 to 50 pore volumes) of fluid during coreflood experiments. Therefore, Haggerty and Seyler (1997) state that their coreflood experiments most closely represent the completely flushed reservoir zones and, under these conditions, the precipitates cannot deposit and cause formation damage in porous media, within the time scale of the convective flow. The acid soak experiments served for the purpose of observing the long-term effects of reactions in unflushed and incompletely flushed zones.
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