The Valdemar field, located in the Danish sector of the North Sea, targets a Lower Cretaceous, "dirty chalk" reservoir characterized by low permeabilities of <0.5mD, high porosities of >20% and contains up to 25% insoluble fines. To produce economically the reservoir must be stimulated. Typically, this is by means of hydraulic fracturing. A traditional propped fracture consists of 500,000 to 1,000,000 lbs of 20/40 sand, placed using a crosslinked seawater-based borate fluid. The existing wells in the field are completed using the PSI (perforate, isolate, stimulate)1 system. This system was developed in the late 1980s as a way of improving completion times allowing each interval to be perforated, stimulated and isolated in a single trip and has been used extensively in the Danish North Sea in a variety of fields. The system consists of multiset packers with sliding sleeves and typically takes 2-3 days between the start of one fracture to the next. Future developments in this area now require a new, novel and more efficient approach owing to new target reservoir being of a thinner and poorer quality. In order for these new developments to be economical an approach was required to allow for longer wells to be drilled and completed allowing better reservoir connectivity whilst at the same time reducing the completion time, and therefore rig time and overall cost. A project team was put together to develop a system that could be used in an offshore environment that would satisfy the above criteria, allowing wells to be drilled out to 21,000ft and beyond in excess of coiled tubing reach. The technology developed consists of cemented frac sleeves, operated with jointed pipe, allowing multiple zones to be stimulated in one trip, as well as utilizing a modified BHA that allows for the treatments to take place through the tubing, bringing numerous benefits. The following paper details the reasons for developing the new technology, the development process itself, the challenges that had to be overcome and a case history on the execution of the first job of its kind in the North Sea, in which over 7MM lbs of sand was pumped successfully, as well as the post treatment operations which included a proof of concept in utilizing a tractor to manipulate the sleeves. Finally, the production performance will be discussed supported by the use of tracer subs at each of the zones.

The Danish North Sea Halfdan Field is developed with extremely long horizontal wells in a line drive producer/injector pattern. The carbonate reservoir requires successful acid stimulation to maximize the flow potential of the wells. High rate, matrix acidizing, relying on maximum pressure differential and injection rate (MAPDIR) was the initial stimulation process used to treat these wells.1 Later, this process was continued with the addition of chemical, in-situ viscosifying diverter stages to improve acid placement. Pre-drilled liner completions have been the primary method across the Halfdan field and has been a very cost effective, efficient way to complete the extended laterals required. Multi-discipline pattern reviews are routinely held to review the producer/water injector performance within the field. These reviews included well flow performance, intervention history, stimulation review, caliper/corrosion history, and 4D seismic interpretation. Candidate wells were identified for possible intervention work, conformance, and stimulation opportunities. Although the liner completions were designed to optimize acid placement, increased volumes of in-situ crosslinked acid, with seawater over flush, was used to improve placement. In addition, trials were developed to also include a particulate diverter material in several producing well restimulation treatments. The pressure response and the production gains were very encouraging and this led to additional re-stimulation candidate selection across the field. This paper reviews the outcome of one of these reviews in regards to water injection performance in some parts of the Halfdan Field. Prior to this project, it was typical to stimulate all water injectors with high rate acid treatments and, in some cases, staged volumes of in-situ crosslinked acid for diversion, along with the pre-drilled limited entry liner. The particulate diverter, used on some producers, was soluble at temperatures higher than the bottomhole temperature of the water injectors. This challenge was addressed during the review and a method agreed on to evaluate the effectiveness of the particulate diverter to improve the acid distribution in these water injectors. A project team worked together to produce a design for the re-stimulation treatments and to develop a methodology to use this particulate diverter in a low temperature environment. The team worked through a procedure to flowback the water injector following the re-stimulation treatment. A risk assessment was completed in terms of flowing back into a donor well and simulations were completed to determine that flowback rates were sufficient to return any solid material to surface. The case history is reviewed to present the design and execution of the two water injector re-stimulation treatments. Following the cleanup/flowback of the wells, water injection was re-established to return to pretreatment bottomhole injection pressures. The results of the water injection rates are discussed as well as further plans for similar treatments.