Within ADCO, horizontal drilling has been used successfully to enhance well productivity, injectivity, and to reduce water coning in some production wells. This paper presents a case history of how horizontal drilling has been used to reduce the effects of water slumping between pseudo-reservoirs, resulting in reduced water cut and increased productivity; and reservoir simulation efforts to match and predict well performance.

A short radius horizontal hole of 850 ft length was drilled in the dry Thamama B Lower pseudo-reservoir in an existing well in the Asab field, and succeeded in reducing water slumping from the wet upper pseudo-reservoir. This resulted in a reduction in the water cut from 20% to less than 1% with a five fold increase in the well productivity. A simulation model of a reservoir element was constructed with local grid refinement to represent the horizontal well. Past performance of the well was matched and the model was used to predict future well performance.


The Thamama B reservor in the Asab field is comprised of two pseudo-reservoirs, both pressure supported by peripheral water injection. Water advance in the upper pseudo-reservoir faster due to greater permeability than that of the lower pseudo-reservoir (Figure 1). A limited natural barrier to vertical flow exists between pseudo-reservoirs and water slumping from the wet upper pseudo-reservoir to the dry lower pseudo-reservoir has occurred. Water slumping has been further increased near producing wells as a result of locally higher pressure differentials.

Performance History

The subject well produced from the upper pseudo-reservoir (1973–1980) until closed-in due to high water cut. The well was recompleted in 1980 to produce from the upper and lower pseudo-reservoir separately. The upper pseudo-reservoir soon ceased flowing due to the high water cut. The well continued to produce from the lower pseudo-reservoir with restricted production rate (and hence drawdown) to delay further increase in the water cut and early cessation of natural flow. The average production rate was 500 bold with water cut increasing to near 20% by early 1990 (Figure 2).

To alleviate water slumping and increase well productivity a short radius horizontal hole of about 850 along the middle of the lower pseudo reservoir (Figure 3). This resulted in the water cut being reduced to less than 1%, and a five fold increase in well productivity (Figure 2). The pressure drawdown decreased from 750 psi in the vertical well to 160 psi in the horizontal at the same production rate.

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