Hot, horizontal open hole limits offshore stimulation options

An operator in the Middle East operates a number of offshore wells. Historically, the operator has drilled vertical and deviated wells, but recently the company began a horizontal drilling campaign to boost oil production from the field.

Because of their high H₂S and CO₂ content, the wells are drilled overbalanced, which normally causes filtercake damage to the producing formation. To bypass the damage and stimulate production from the carbonate formation, the operator typically uses conventional matrix acid treatments, bullheading 15% HCl at 10 to 15 galUS/ft [124 to 186 L/m] from the offshore rig. Although the bottomhole temperature reaches 230 degF [110 degC], the cooldown effect of the stimulation fluid volume in the relatively short vertical interval minimizes the risk of rapid acid spending.

For the next horizontal well, a new stimulation approach was required for several reasons related to the 1,400-ft [427-m] openhole section. First, bullheading was unlikely to distribute the fluid across such a long lateral. Second, the volume of HCl to treat the section would require a dedicated stimulation vessel to support the operation because of the limited footprint on the offshore rig. In addition, the time to pump such a volume would be expensive in terms of rig operating time. Finally, the longer openhole section would limit the cooldown effect, likely leading to rapid HCl spending and face dissolution rather than deep wormhole penetration.

For the Arabian Gulf well, the acid system’s efficiency enabled skin reduction with a dosage of only 5 galUS/ft through CT. The reduced acid volume and limited equipment footprint eliminated the need for a stimulation vessel. In addition, simulations indicated that pumping the smaller volume would save 3 hours of rig time compared with pumping the larger HCl volume.

To model CT reach and deliverability, engineers used CoilCADE coiled tubing design and evaluation software. Also, upon final fluid selection, formation fluid samples were tested with the engineered acid system and additives to verify compatibility.

Treatment saves money and stimulates oil production

The OpenPath Reach service operation was conducted as approved by the operator, with 1 ¾-in CT reaching total depth to distribute the single-phase acid system throughout the open hole. In all, the treatment squeezed approximately 3,960 galUS [15,000 L] into the formation from toe to heel.

The resulting oil production exceeded the operator’s expectation by more than 1,000 bbl/d [159 m³/d]. In addition, reducing the job time and eliminating the need for a stimulation vessel saved the operator more than USD 180,000.

The operator has decided to replace 15% HCl with the single-phase acid system in upcoming stimulation operations.