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Composition and distribution of clays (both as clay minerals and true shales) can have a direct impact on the day-to-day and long-term economic performance of a well. Clays affect the following:
(1) Distribution and quantity of interstitial fluids (i.e., value of reserves in place). The specific composition of dispersed (authigenic) clays controls the surface area of the pore system. Water bound in micropores created by these dispersed clays can comprise ±70% of fluid volume, but the well may produce water-free hydrocarbons. In such shaly sands, calculation of Sw by itself often may not identify either the amount or type of fluid production. It can predict the presence of hydrocarbons, but is not an optimum indicator of fluid production.
(2) Rate of production (i.e., time value of reserves). The distribution of clays affects rate of production. Laminar clays will not reduce flow rates as significantly as dispersed clays. For example, a sand containing 20% clay distributed as laminae will have its net pay reduced by 20%. Production from the sand laminae is not affected by the clay. However, a sand with 20% clay dispersed in 30% pore space may not produce economic quantities of hydrocarbons. Furthermore certain clay minerals, such as fibrous illite, can significantly increase flow tortuosity and reduce daily flow rates.
(3) Wire-line log response (i.e., bypassed production). Calculated values of Sw, porosity, and Vshale must be interpreted considering clay type and shale distribution. High values of log derived Sw can indicate (a) highly productive laminated sand-shale sequences, (b) low permeability dispersed clay production, or (c) water production. Calculated porosities depend upon assumptions of rock density, which can be significantly altered by the presence of shales. Vshale is calculated from the gamma ray response, yet three of the four major families of clay minerals are nonradioactive and do not have any effect upon gamma response. Much production is bypassed due to inadequate knowledge of clay composition in potential horizons.
(4) Completion. The role of clay minerals on completion procedures is well documented. Clay minerals dispersed in pores can interact with common well-bore fluids and irreparably damage potentially productive sands. A knowledge of detailed clay compositions is vital to successful stimulation effects. Individual wells or whole zones can be written off as nonproductive if inappropriately designed stimulation efforts prove unsuccessful.
Identification of bypassed production and incorrectly stimulated zones are particularly important during field development because of the heavy front-end investment necessitated by development. Once this investment is made, then hydrocarbon-productive zones that were not obvious or considered uneconomic during exploratory evaluation, become important targets. Experience suggests that much bypassed production contains unusual clay content or clay distribution.
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