A field in Makassar Strait, Indonesia has been producing gas for 5 (five) years and it has shown a decrease of production in some wells. This decrement in well performance was confirmed by well test data, which highlighted the presence of skin, and hence the requirement for well stimulations. The low reservoir pressure creates challenges for stimulations process, while the subsea environment calls for a cost-effective approach. Several solutions were applied to stimulate highly depleted gas wells in bull-heading without retrieving the crown plugs.
The standard way to perform intervention work in this field is by having a drilling rig or intervention vessel to retrieve the crown plugs to have well access. This approach implies complex operations and involves many technologies such as slickline, wireline, and even coiled tubing package. To have a more cost and time effective approach, the acid treatment has been performed by bull-heading with a flow-path through the X-Tree crossover loop then down into formation.
The pumping job was therefore performed using an available rig, in continuation of a sidetrack campaign in the same field, taking full control of the X-Tree. This set-up consists of a current pumping package (liquid and nitrogen) in the rig and 5.5″ drill pipes were used as the landing string to connect to the subsea tree.
HCl-HF acid was selected as the main stimulation fluid for this sandstone formation, based on X-ray diffraction data from the reference well, reservoir properties and skin estimation. All the stimulation fluids were tested in the lab against all elastomers and metal components of the subsea tree that would be exposed to the flow, confirming no issues were to be expected.
Due to the depleted reservoir pressure, the other challenge was to limit fluid volume and lower the fluid hydrostatic to reduce the risk of accidentally killing the well. Based on the results of detailed simulations, this was obtained by nitrifying all the stimulation fluids and performing the final displacement with nitrogen, thus giving proper underbalance to restart the well at the end of the job, without the need for lifting via coiled tubing.
The treatment was performed safely without any failure in the surface equipment, drill pipe landing string or the subsea tree components. When the acid treatment was completed on two pilot wells, the wells were successfully restarted, despite low reservoir pressure and injected fluid, and flowed back directly to FPU. As a result of the acid treatment, the well performance showed improvement which indicated successful skin reduction. Both wells are currently producing as per target and beyond.
This work serves as an example for how to qualify the well material (tubing and X-Tree equipment) for an acid stimulation. This well stimulation employ a nitrified HCl-HF acid pumping technique to treat a sandstone formation with depleted reservoir pressure while considering the flow assurance challenges (e.g., hydrate formation) in deep water environment. In addition, the pumping strategy that did not involve the retrieval of crown plugs proved to be an efficient and cost-effective approach in a challenging subsea environment.
