Sand and proppant production pose a safety risk due to erosion, fill of wells and facilities, often resulting in significant deferred production. A number of wells in the Danish offshore sector are currently closed in or beaned back due to proppant production from sand propped fractured wells where proppant is back produced to surface facilities which were not designed with sand handling capability. A new sand consolidation treatment involving enzymatic calcium carbonate scale has been applied to individual zones downhole to remediate failed proppant fractures. The technology is an environmentally friendly alternative to commonly used resins and has the added benefit of being completely reversible. A detailed coiled tubing program was successfully executed in a harsh offshore environment with numerous challenges including identification of sanding zones, chemical contamination, logistics, and selective downhole placement. Laboratory testing was undertaken where unconsolidated proppant was treated with the consolidation chemicals. These results provided important input for defining the placement strategy and indicated that results could be replicated in the field. Successful results have been achieved from this industry first application of enzymatic calcium carbonate scale to consolidate sand propped fractures in a chalk reservoir. The field application supports the laboratory results, where sand free production of over 700 BOPD has been restored in a well previously closed due to proppant production with limited impact on well productivity. A post-job monitoring program has been designed to further evaluate this technology. The development of enzymatic calcium carbonate scale consolidation has led to a method for chemical consolidation of proppant fractures that is more environmentally friendly than alternative methods, is reversible and has limited impact on well productivity. Introduction The Gorm field is a chalk reservoir located in the Danish Offshore sector, created by salt uplift (Nederveen and Damm 1993). It produces from the Danian and Maastrichtian formations via approximately 30 active mainly horizontal production wells. The oil producers are generally completed using the Perforate, Stimulate, Isolate (PSI) concept (Damgaard et al. 1992). Pressure support is provided by four vertical and seven horizontal basal water injectors. The reservoir is tight, with matrix permeability of around 1 mD. The wells are either stimulated by means of matrix acidisation, high rate acid fracturing or sand propped fractures. Proppant and sand are used interchangeably throughout this paper, with both terms referring to the sand based proppant used during stimulation. Three key challenges to production are; scale formation (both barium sulfate and calcium carbonate),pressure maintenance due to irregular waterflood patterns and water short cuts, andsand production from de-stabilised fractures.
This paper will discuss the design, qualification, testing and planning of a new intervention conveyance technology used to successfully remediate a failed down hole safety valve (DHSV). The technology is a high pressure, reinforced, ¾ inch OD hose that is run on standard wireline equipment thereby reducing footprint, manning, cost and job time compared to other intervention methods. The hose has a working pressure of 12,500 psi with a tensile strength of 14,300 lbs. The system is gravity-fed into the well similar to a conventional wireline system and is spooled onto a wireline add-on drum with conventional wireline pressure control equipment (PCE). The difference compared to a standard wireline equipment is a modified add-on drum which includes a swivel for pumping, a purpose built stripper installed above the lubricator, and special inserts for the BOP. Before mobilisation a series of tests were carried out to qualify the reinforced high pressure hose to ensure it was safe for interventions. Based on successful test results, the coil hose was mobilised and a total of 5 runs were carried out on the first job. These runs included jetting, spotting of chemicals and a caliper survey. The intervention successfully jetted 15% hydrochloric acid (HCl) over the DHSV, and through to surface. The job was an operational success and the DHSV inflow test, post treatment was successful. The reinforced, high pressure coiled hose is a new conveyance technology that positions itself between wireline and coiled tubing. This technology can access wells on locations where running coiled tubing (CT) operations can be a challenge due to limited crane capacities and/or deck space. While coiled hose is not a replacement for CT due to its limited pumping rates (approx. 50lpm) and gravity feeding, it is suitable for spotting chemicals, N2 lifting and light cleanout operations.
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.
A coiled tubing (CT) campaign was executed on the Skjold field in the Danish sector of the North Sea in 2014. One of the objectives of this campaign was to perform production logging in horizontal oil producer wells, located in a highly fractured chalk reservoir. Most of these wells are unable to flow without artificial lift due to the high water cut and low reservoir pressure. Stable flow for logging was achieved by combining CT operations with a surface Liquid Jet Compressor (LJC). The campaign was successful in acquiring both production (PLT) and spectral noise logging (SNL) data. The SNL tool complemented conventional PLT as it provided information on quality of zonal isolation, vital in planning of water shut off and/or re-stimulation activities. Well A was completed with a gas lift valve straddle which had to be retrieved to gain access to the wellbore for production logging. Retrieving the straddle disabled access to lift gas, which meant the well was unable to produce. Liquid jet compressor technology in combination with nitrogen lifting through CT was implemented to allow steady fluid flow for logging. The surface liquid jet compressor (LJC) lowered the production system backpressure seen by the well from 500 psig to 0. Together with the continuous nitrogen pumping through CT, stable well flow was achieved and the logging program was successfully executed. The results were used to identify fluid flow behind casing, water producing zones and to identify sub-optimally producing oil zones for re-stimulation. Using the LJC significantly reduced the total nitrogen volume required by 60%. This was crucial as limited crane capacity and deck space presented additional logistical and operational challenges. In order to overcome these, equipment had to be rigged up on a bridge connecting two platforms with the CT reel lifted in three spools and welded on location. This in turn required the fiber optic cable to be injected on-site offshore. The novel combination of a surface LJC with nitrogen lifting via CT reduced the number of crane lifts, load outs, nitrogen transfers and overall cost. Furthermore, it improved operational efficiency and job safety. Ultimately, this combined concept enabled successful acquisition of the required data.
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.
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