The Netherlands is home to underground gas storage wells that are used to store gas during low-usage summer months, and used to produce gas during peak delivery periods throughout the winter. To ensure that their contractual obligation to deliver adequate gas supplies during the peak period is met, NAM aimed to increase the production and injection rate of one of its 7-5/8" gas storage wells, GRK-47, in Grijpskerk. In order to achieve this objective, NAM retained BJ Services to carry out a coiled tubing intervention operation on the well, which required acid stimulation to restore the well's capacity to six million scm/d.
Preliminary Assessments Key to Success
As BJ in conjunction with NAM assessed the well for the acid screen wash and mud-acid squeeze, several concerns were expressed about the impact of such an operation:
-The acid resistance of the pre-packed screen, mesh wire and pre-packed Bakerbond sand.
-Potential damage to the mesh wire by the jetting impact during the screen wash.
-Potential damage to surface equipment, given the presence of oxygen and chlorides in the treatment fluids.
To address these concerns, BJ carried out several corrosion tests on samples of the actual screen using the proposed treatment recipes at its laboratory in Tomball, Texas. By doing so, BJ established the required corrosion inhibitor loadings for the acid recipes. In addition, an investigation into the integrity of the mesh wire and pre-packed sand was carried out. Given the shape of the mesh wire, the weight loss per square foot would be extremely difficult to determine, so a microscopic visual examination was performed to evaluate the integrity of the mesh wire following a 12-hour soak in the acid at bottom hole temperature. Visual inspection of the mesh wire after the acid soak at a magnification of 120 times showed no damage to the screen. The integrity of the sand was examined by subjecting it to Gravimetric Solubility tests, with acceptable results.
Having established the appropriate acid recipe, the impact of the jetting force on the screen was investigated. To reduce the impact pressure on the screen, but retain optimum screen coverage, BJ considered using its patented Roto-PulseSM tool. Because the treatment would be pumped using nitrified fluids in order to remain within collapse limits of the coiled tubing, the Phase Separator was introduced to further reduce the impact pressure by separating the nitrogen from the fluid flow just above the Roto-Pulse tool in the well.
A test was carried out in the yard whereby four sections of pre-packed screen were jetted with water using a variety of different jetting nozzles: the Roto-Jet® tool, Roto-Pulse tool, a 45-degree and a 90-degree jet nozzle. As a 7-inch pre-packed screen was not available, the test was performed using a 4-1/2-inch, but similar, pre-packed screen. Although the actual operation was only programmed for four passes, the preliminary tests were performed with eight passes while water was pumped at 150 liters per minute.
The tests confirmed that no damage was sustained by the mesh wire or to the pre-packed sand during the tests using the Roto-Pulse and jet nozzles. Although the mesh wire in the screen jetted by the Roto-Jet tool didn't exhibit any damage, the pre-packed sand did appear to be washed at several spots in the screen. Based on these results, NAM agreed to the use of the Roto-Pulse tool.
Preventing Oxygen in Produced Fluids
To carry out the acid treatment, BJ used the following bottom hole assembly:
CT Operation a Success
Production Rate Up 2 Million SCM
"Given the dramatic increase in production and injection rate of this well, it illustrates the impressive impact that the acid stimulation process has." said Neil Gordon, region manager – Europe & Africa Region-Well Services for BJ Services. "We're extremely pleased that we achieved - even exceeded - the objective to restore well capacity. This will help NAM most certainly to meet demands for gas in this region during peak periods," he added.
BJ's base in Emmen, The Netherlands, provided all related equipment, including the CT unit, fluid pump and nitrogen unit, as well as the operational team to carry out the operation successfully.
The operation on NAM's Groningen field was carried out by BJ Services as the result of a five-year contract awarded earlier this year by Shell Exploration and Production in Europe to act as the company's sole supplier of coiled tubing, stimulation and nitrogen services throughout Europe.
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