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The Upper Cretaceous Woodbine Formation contains diagenetic components in the form of cements and clays which can cause problems in drilling, completing, and stimulating a well. These diagenetic components are present in pore systems of the rocks deposited within various Woodbine depositional systems ranging from fluvial to deep marine. Fluvial environments were present in the northeast area of the East Texas basin, and changed to deltaic-marine systems to the south and southwest. Deeper marine sediments are represented by a thickening clastic wedge deposited over the Edwards reef trend as channels, interchannels, and submarine fans. It is necessary to identify mineral types, crystal morphologies, and modes of occurrence of the diagenetic components within pore systems of rocks formed in these various depositional settings so that proper drilling, completing, stimulating, and/or acidizing programs can be conducted.
Calcite, dolomite, ankerite, and quartz are important cements which reduce porosity and affect reservoir quality of the Woodbine Formation. Carbonate minerals occur as isolated patches or extensive cement within the intergranular network. Quartz cement is commonly observed in the form of euhedral overgrowths. The storage capacity of the reservoir and productivity of a well can be hindered where these cements reduce and isolate primary and secondary pores. During completion, stimulation, and possibly acidization, calcite may cause further problems through reaction with hydrofluoric acid and precipitation of formation-damaging calcium fluoride. Iron hydroxide precipitates may also form when iron-rich calcite, dolomite, and ankerite are contacted by HCl, HF, and HCl/HF acids. These preci itated gels can block pores and reduce production.
Important clay components found within the pore network of the Woodbine Formation are kaolinite and chlorite. Kaolinite commonly displays a pseudohexagonal "book" and platelet morphology. It is relatively stable with respect to acids; therefore, acidization should have minimal effect on the kaolinite. A problem of migration of fine particles can arise when these "books" and platelets are loosened from framework grain surfaces. Turbulence within pore networks, caused by fluid movement during stimulation and production, especially near the wellbore or a fracture face, can cause the kaolinite fines to move and block pore throats. This could result in formation damage. Chlorite occurs as well to moderately crystalline platelets which reduce porosity by lining and filling pore areas. If th chlorite, is iron-rich and contacted by HCl, HF, and HCl/HF acids, a problem of iron hydroxide precipitation can occur.
Other clays within the Woodbine Formation include illite and smectite. Authigenic illite is found as incipient growths on chlorite platelets. The smectite has a honeycomb morphology and occurs as a grain coating. These components can cause problems if present in significant amounts within Woodbine reservoirs. If relatively fresh water is allowed to contact the formation, illite can "mush" and the smectite can swell, both damaging the formation.
Drilling, completing, stimulating, and acidizing programs can
be designed to minimize problems caused by diagenetic components. Well production can even be increased when proper procedures are designed using information on potential formation damaging diagenetic minerals that are present within pore systems of Woodbine reservoir rocks.
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