Thousands of wireline conveyed perforating jobs are executed every month around the world; however certain jobs have a higher risk of weak-point breakage due to dynamic pressure loads, known as gunshock loads. Gunshock loads result from pressure waves in fluids and stress waves in structural components. Perforating under all conditions (i.e. static/dynamic overbalance or underbalance) can produce pressure waves and/or reservoir surge of large magnitude leading to wireline weak-point (WWP) failures and/or cable damage. These risks are assessed as part of the job preparations. In this paper we focused on Dynamic Underbalance (DUB) because perforating with DUB can deliver clean perforations with very low risk of gunshock damage when properly planned. For any perforating job on wireline, the magnitude and duration of pressure and stress waves depend on job parameters that can be adjusted, such as type and size of guns, shaped charges, gun loading layout, wellbore fluid, placement of packers and plugs, and cable size. For perforation damage removal we need a job design to generate a DUB of enough magnitude, using the right gun types and loading to produce a DUB of large-amplitude but short-duration, thus removing perforating rock damage while minimizing gunshock loads on the WWP. Perforating job designs are evaluated with software that predicts the transient fluid pressure waves in the wellbore and the associated structural loads on the cable and tools. All aspects of well perforating are modeled including gun filling, wellbore pressure waves, wellbore and reservoir fluid flow, and the dynamics of all relevant solid components like cable, shock absorbers, tools, and guns. When planning perforation jobs that may have a significant risk of weak-point breakage, we predict the peak dynamic loads on the cable and weak-point during the design process, and when necessary we make design modifications to reduce the peak load on the WWP. The software’s predictive capabilities are demonstrated by comparing downhole fast gauge pressure data (110,000 data points per sec), shock absorber deformation, and cable tension logs with the corresponding simulated values. Fast gauge pressure data from perforation jobs shows that the software predictions are sufficiently accurate to evaluate the gunstring dynamics and the associated peak tension load on the WWP as part of the job planning process. Residual deformation of shock absorbers correlate well with predicated peak axial loads at the WWP, and available cable tension logs from vertical wells show that the cable surface tension is well predicted. The simulation software described in this paper is used to minimize the risk of unexpected release of tools and guns due to perforating dynamic loads, thereby minimizing the probability of non-productive time (NPT) and fishing operations.
Oman completion strategies continue to evolve as more cased horizontal wells are completed. These wells must be perforated and, for the best production performance, should be completed underbalance avoiding any kill as part of the process. Early wells were perforated using coiled tubing with mechanical sealed ballistic connector 1 to shoot in underbalance conditions and retrieve the gun assembly without having to kill the well. While the use of Coiled Tubing is a very effective deployment method, the extreme trajectory of some wells requires that fluids to ease drag friction be pumped as part of the deployment process. These friction reducing fluids may damage the perforations resulting in poor productivity.A new wireline tractor conveyance technology designed for the harsh, high shock application of perforating has been used to perforate a long horizontal well section in underbalance conditions. Detailed pre-job planning and modeling was carried out to ensure the job could be executed as desired. Also the "lessons learned" gained in a previous attempt to use this technology helped improve the delivery efficiency. The guns were deployed and retrieved under controlled conditions minimizing any perforation damage in addition to significantly reducing the running times compared to other conveyance methods used in previous wells.Eighteen successful tractor runs were made deploying up to 18.9 m of 2 7/8" OD perforation guns per run shooting 253m of perforation interval. Operating time was reduced significantly compared to previous perforating operations and no additional formation damage was done to the perforations as part of the process. Subsequent well productivity was better than expected.The success of this operation opens up a wide range of applications for perforating or other heavy duty and high shock applications for this tractor technology. In the future different deployment technologies can be combined with the tractor for conveyance to push even longer gun strings into highly deviated wells in "live well" conditions further improving completion efficiency.This paper will review the technical considerations and the detailed planning exercise that is required for a successful tractor perforating job as well as the operational requirements to minimize nonproductive time (NPT). This in turn will improve job planning and the likelyhood of operational success for future perforating jobs in this complex operating environment.
Well productivity is driven by establishing a clean connection through the near wellbore zone of drilling and completion induced permeability impairment commonly referred to as the "near wellbore damaged zone". This connection through the damaged zone is most often achieved by perforating with explosive shaped charges. The effectiveness of this connection is the result of perforator selection criteria and the well environment in which the perforating job is executed. In the depleted oil field under study a typical completion is perforated using large Tubing Conveyed (TCP) guns in "shoot and pull" mode in a static underbalance environment. After shooting the well is killed to allow the TCP guns to be pulled safely and the completion run. Production from the newly perforated intervals declines quickly due to near wellbore damage caused by the kill cycle. The challenge in this case was to identify the proper perforating gun system, conveyance method and perforating program to achieve the optimal productivity once the well is put on production. Modeling software was utilized to predict the productivity ratio (PR) for different perforating systems considering gun size, charge type, shot density, reservoir parameters and the well conditions at the time of shooting. The perforating program was modified to perforate the well with gas lift on in a flowing condition maintaining under balance conditions after shooting to assist with cleanup. The under balance at the time of shooting was managed using a permanent down hole gauge installed in the completion string. The first well completed using this process showed improved production of greater than 3 times what was expected when compared to similar wells in the field. This paper will cover the job design criteria, the job execution requirements and evaluation of the results. In addition, a summary of the study leading to this work and the total cost reduction details for the completion operations is also included. The result of this novel perforating job design has led to a new completion strategy for oil fields in Oman thus improving overall well performance. Introduction A key element of how productive a well will be is the effectiveness of the perforations. The perforation creates the path for the formation fluid to flow from reservoir to the wellbore. When the perforations are inadequate or plugged off inflow will suffer over the life of the well. The quality of the perforation job design and execution is a major concern in any completion design 1,2,3,4,5,6. An effective perforating job is dependent on many factors including:Perforating charge performance which is a function of reservoir rock strength, effective stress, fluid type and completion parametersPerforating gun characteristics such as gun size, shot density, phasing and charge typeWell conditions at the time of perforating including underbalance and fluid type in the wellboreWhat happens to the well post perforation e.g. a kill cycle which may damage the perforations and reduce productivity.
We present perforating on wireline with dynamic underbalance (DUB) to simultaneously maximize productivity and minimize gunshock. We focus on perforating on wireline with DUB because, when compared with other approaches, perforating with DUB is probably the best method to deliver lower tunnel plugging and lower formation rock damage, with lower risk of tool damage due to gunshock or guns blown uphole. Specifically, we present two important aspects of perforating on wireline using DUB: prediction of wellbore dynamics to assess perforation tunnel and formation cleanup and gunshock prediction to assess the risk of tool damage. We present the latest models used to evaluate perforating jobs for well productivity and for operational risks.It is well known that the DUB produced when perforating with the right gun system can remove formation rock damage and tunnel plugging produced by shape charges. What is not so well known is how much DUB (amplitude and duration) is necessary, and how to predict how much DUB will be generated by a gun system. To achieve formation tunnel cleanup, we need a DUB of large amplitude but short duration to remove perforating rock damage and plugging while minimizing gunshock loads. In the pre-job design, we simulate/predict the transient fluid pressure waves in the wellbore and formation rock to predict formation rock damage cleanup and also the associated gunshock loads. DUB amplitude and duration depend on job parameters that can be adjusted, such as type and size of guns, loading of standard perforating charges and DUB charges, and placement of packers, if present. Important physics included in the model are: gun filling, wellbore pressure waves, transient reservoir fluid flow, and the dynamics of all relevant solid components (cable, shock absorbers, tools, and guns).The reliability of the DUB prediction model is demonstrated by comparing downhole fast-gauge pressure data with the corresponding simulated values. When the reservoir properties are well known, the predicted DUB amplitude and duration are very close to the field data values, typically within 15% or less. The reliability of the gunshock loads is demonstrated with residual shock absorber deformation and cable tension logs. We also demonstrate how gunshock simulations have been useful to explain equipment failures due to gunshock loads.Reliable predictions of wellbore dynamics, transient reservoir flow, and gunshock loads enable operators to select perforating equipment capable of removing perforating formation damage and reduce the risk of unexpected release of tools and guns due to dynamic loads, thereby minimizing the probability of nonproductive time and fishing operations.
In a multi-layer heterogeneous carbonate reservoir in which production and injection is being carried out, one of the biggest challenges faced is to produce/inject evenly from/to the different layers. At elevated temperatures this heterogeneity limits the success of conventional stimulation techniques. A novel acid diversion technique involving selective stimulation and tailored perforation design techniques was applied in an offshore UAE well to improve injectivity in a multi-layered heterogeneous carbonate reservoir.The well in question was re-completed from a dual string water injector well to a single string gas injector re-perforated in four different sub-zones. Well test and production logging data were used to estimate permeability and flow profile before the workover. These data were then used to run NODAL analysis, stimulation simulation, and perforation simulation models. The final workover proposal called for:• Perforation over the same interval as was previously perforated using different gun and charge sizes to establish controlled injection based on the different zones permeability. • Optimized coiled tubing (CT) selective stimulation using high strength retarded acid pumped in stages with a self diverting acid.The well injection performance was dramatically improved as evidenced by the gross injection value and the injection profile obtained in a post treatment production logging run.This paper details the planning, execution and final evaluation of this job which is unique in that the same interval was tested twice, before and after workover, after being completed using different perforating and stimulation techniques. BackgroundCarbonate reservoirs tend to be complex and often exhibit large variations in permeability over different sections of the same producing zone 1 . These variations in the permeability profile create challenges when planning to inject acid (commonly done in carbonates), produce hydrocarbons, or inject water or gas. It is often the high permeability zones that will dominate while the lower permeability zones will miss the treatment or not be produced.
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