Observations on Effects of Lithology on Well Failures

By

WACH, GRANT D.

ChevronTexaco, Houston, TX,

KUHFAL, DENNIS L.

ChevronTexaco, New Orleans, LA

The sediments of the McAllen Ranch field were deposited within shelf-margin deltaic depositional systems. Four main lithofacies were identified from the cores of the Texaco Guerra #68 well. One common lithofacies is a laminated shale interpreted to represent pro-delta deposits. The clay mineralogy of the shale includes a significant percentage of chlorite-serpentine which is very prone to slippage. The differential pressure at which slippage can occur is related to the angle of the fault or deposition. These pro-delta shales were initially deposited on a slope that contributes to their affinity to slip.

Subsurface geotechnical maps can be constructed to identify “hazard” zones by illustrating where the faults exist and where lithology-dependent slippage is of greatest risk. These maps are intended to lower the risk of well failures by improved well placement. Significant savings can be incurred even if the risk is lowered only 10% to 20% by use of these maps. It is also proposed to map regional stress fields based on regional fault trends, key sequence stratigraphic intervals and depositional environments to identify and classify areas of susceptibility to well failure, for example why well failures are common in some South Texas field areas, like McAllen Ranch.