Monday, July 31, 2017

Well Testing Design and Analysis chapter 2

First comes wellbore storage, which refers to the obfuscating role of the wellbore fluid when a transient is initiated. The moment a well is shut in or allowed to flow, fluids in the wellbore must first compress or expand before formation fluids can react. If flow is controlled from the surface, the entire well's fluid contribute to wellbore storage and the effect can dominate the pressure transient for hours afterward. The effect is exacerbated if well pressure toward the top of the well drops below bubblepoint and part of the well is filled with compressible gas. Wellbore storage is substantially reduced by shutting in the well downhole, minimizing the volume of fluids that contribute.

Estimating near-wellbore damage, reduce it if necessary with a matrix acidization and the checking that the acid cleanup worked. This was intended not only to benefit production in the well, but also to help plan a completion strategy for field development. The other goal was to investigate reservoir volume and identifiy reservoir boundaries.

As wellbore storages disspates, the transient begins to move into the formation. Pressure continues building up, but at a slower rate as the transient moves far enough to achieve radial flow toward the wellbore. This is the so called radial-flow regime that appears as a straight line trend on the Horner plot. The radial-flow regime is crucial to quantitative interpretation, since it provides values for kh and skin, S, a measure of the extra pressure drop caused by wellbore damage. Skin takes positive values in a damaged well when pressure drop near the wellbore is greater than expected and negative values when stimulation creates less pressure drop. Next, the transient encounters the limits of the reservoir and pressure departs from its straight-line radial flow response.

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