Long horizontal wells are hydraulically fractured or acid stimulated to economically develop chalk reservoirs in the North Sea. To separate the zones and ensure conformance, each fracture or zone is separated by isolation packers and flow is controlled with a sliding sleeve. With time, conformance control capability of the lower completion may be lost if the sliding sleeve is stuck open or holes are eroded or corroded in the lower tubing. If a shortcut has developed between an injector and producer, either during stimulation or operation of the well, the consequence of losing conformance capability of the lower completion is more severe. Another scenario with serious consequences occurs when the well has been hydraulically fractured with proppant and the proppant is back-produced without a possibility to close off the zone. To mechanically fix this conformance issue, it would require working over the well and replacing the lower completion. In an old (30-40 years) well, this is a challenge. A better solution is to solve the problem chemically by consolidating the sand in the fracture or plugging the fracture connecting the injector and producer. Recovery from the reservoir will then be optimized and the solution is likely to be cheaper than working over the well. Enzymatic CaCO3 technology is a novel chemical system that can be used for plugging fractures and consolidates sand in propped fractures in horizontal wells. All chemicals used offshore in the North Sea need to be compliant with OSPAR regulations; the Enzymatic CaCO3 system is classified as environment-friendly. It also has predictable chemistry and is completely reversible. This paper discusses the process of maturing the technology from concept to readiness for field implementation covering lab testing, IP protection, chemical procurement strategies, environmental impact evaluation, and quality assurance process during field application.

For the first time in a Danish field, micro-fine cement was used to abandon a multi-zone horizontal well by plugging each sand fractured zone sequentially. Lab testing was performed to investigate the depth of penetration of different conformance systems in a 20/40 sand pack. The micro-fine cement showed the highest penetration and was considered the best option for abandoning sand propped fracs in horizontal wells. The first field application was unsuccessful due to pre-setting of the micro-fine cement in the Coiled Tubing. Extensive lab tests were performed to understand the causes of the pre-mature setting of the cement. A new cement recipe was selected and subjected to modified lab test procedures to successfully abandon the horizontal well. To economically develop chalk reservoirs in the North Sea, long horizontal wells are drilled, hydraulically fractured and waterflooding is applied to the field. To ensure conformance, flow from each fracture is controlled by a Sliding Side Door (SSD). Well A suffered injection water breakthrough which could not be controlled by closing the SSDs. To enable the full abandonment of each zone the decision was made to isolate each zone sequentially from the toe of the well up with cement retainers and to use Coiled Tubing to squeeze a pre-determined volume of special micro fine cement into the zone in order to plug the tubing, the tubing-liner annulus, any cement channels and a limited section of the sand propped fracture. In this manner any potential water channeling between zones and with other wells in the pattern would be prevented. This paper discusses the results of initial lab tests, causes of failure of the first job, testing of new micro-fine cement and the successful application. This paper also discusses the results of computational fluid dynamics simulations which were performed to understand the behavior of cement in a fracture and enable further improvements to similar abandonments in the future.

The application of enzymatic generation and precipitation of calcium carbonate for the use as in-situproppant consolidation is an environmentally green technology that shows promise in the Danish North Sea. Hydraulic fracturing of producers with the aid of proppants to keep fractures open can present a problem if the proppants are back-produced as this can cause erosion of the well and production facilities. These problems may result in delayed production and costly workovers. Enzymatic calcium carbonate, a technology to consolidate the proppants, can be applied right after a hydraulic fracturing treatment or at a later stage, when proppant reconsolidation is needed. In this paper, we report the findings of extensive lab experiments, under reservoir conditions, on consolidation of of three different proppant types. The results show that some proppants are easier to consolidate than others.