We present results of a systematic study aimed at the identification of fundamental characteristics of wormhole formation in perforated core experiments and the determination of their underlying dependence on perforation properties. We performed a set of single-phase laboratory experiments in which medium- permeability (10–20 mD) and low-permeability (< 3 mD) Indiana limestone cores were perforated by conventional shaped charges to produce worst-case damage conditions. Then, those cores were stimulated with 15% HCl until wormhole breakthrough. We observed strong characteristic differences in the evolution of pressure drop during acidizing, required acid volume, and resulting wormhole patterns with changes in initial core permeability. Notably, all tests in medium-permeability cores showed a "transverse wormhole" mechanism in which the dominant wormhole nucleates behind the tunnel tip, propagates perpendicular to the tunnel, and then turns and propagates along the core axis upon approaching the no- flow boundary. In contrast, tests in low-permeability cores showed dominant wormholes nucleating directly at the tunnel tip. We found that the acid volume required for breakthrough scales linearly with the axial distance between the dominant wormhole nucleation site and the back face of the core. Mechanisms underlying transverse wormhole formation in the acidizing experiments were then identified and analyzed with a novel characterization and simulation workflow, which attempts to connect the initial state of the tunnel to the observed acidizing outcome. Image analysis of whole-core computed tomography (CT) scans extracted the initial tunnel geometry and identified the presence of debris and perforation- induced fractures prior to stimulation. CT analysis suggests that transverse wormhole nucleation sites coincide with the presence of secondary radial fractures at the tunnel wall that are impregnated with liner debris and protrude partially into the crushed zone. Thin section analysis was then employed to quantify the spatial distribution of crushed zone and virgin rock permeability. The tunnel geometry and thin section characterization data were combined to estimate the leakoff velocity profile along the tunnel using an approximate analytical solution for the flow field in the crushed zone. The leakoff velocity shows strong axial variation along the tunnel, with local maxima in the vicinity of dominant transverse wormhole nucleation sites. Local peaks in the leakoff velocity are shown to coincide with locations along the tunnel that have elevated levels of crushed zone permeability stemming from a lesser extent of macropore compaction in the near-tunnel zone. Finally, we present a continuum acidizing model, which extends the two-scale continuum (TSC) approach of Panga et al. (2005) to explicitly model nonuniform perforation and crushed zone geometries. Numerical simulations of the prestimulation flow field and of wormhole formation in perforated cores suggest that the shift from transverse to tip wormhole nucleation with change of initial rock permeability is fundamentally related to the increasing influence of the no-flow lateral boundary as the crushed-zone to virgin-rock permeability ratio increases by an order of magnitude from medium-permeability to low- permeability cores.

Many large wells have been drilled in the Gulf of Mexico's Lower Tertiary play. These wells are completed with single-trip multizone systems, and they have gross perforated lengths exceeding 1,500 ft. The main difficulty in perforating these wells is the high-pressure environment (~20,000 psi). Under these conditions, the challenges are to create sufficiently large entrance holes in the casing, minimize the high-risk of equipment damage due to gunshock, and minimize the amount of perforating debris created. Perforating several intervals in a single run is required to complement single-trip multizone systems. Perforating all zones simultaneously in one trip saves time and reduces risks when compared with stacked completions requiring multiple trips for each zone. Safety and cost reduction are extremely important in deepwater operations. Risk control is very important because gunshock and/or debris problems can lead to multimillion dollar losses in non-productive time, and in extreme cases, gunshock problems can lead to lost wells. To undertake these challenges, a new Low Perforating Shock and Debris (LPSD) gun system was used. In comparison with standard high-pressure guns, the LPSD gun system produces much less gunshock and negligible amounts of debris; thus, minimizing gunshock risk and reducing cleanup runs typically needed to recover perforating debris. LPSD guns produce negligible amounts of debris because LPSD guns contain all the metallic components, including the shaped charge cases, which remain virtually intact inside of the guns. A key element in planning these perforating jobs is gunshock prediction to evaluate if the equipment will be able to withstand the transient loads produced by the perforating guns. The gunshock prediction process is described in detail in this paper. For a typical 4-zone 1,500 ft gross length perforating job, the time needed from picking up the first gun to laying out the last gun averages 84 hours. All zones are simultaneously perforated, which eliminates three perforating runs per well, saving approximately 9.2 days per well while minimizing personnel exposure. By perforating the largest high-pressure wells in the Gulf of Mexico's Lower-Tertiary play with LPSD guns, we minimized personnel exposure, minimized debris and reduced execution time up to 72%.

The largest deepwater wells in the Gulf of Mexico have been drilled in the Lower Tertiary play. Most of these wells are completed with single-trip multizone systems have gross perforated lengths exceeding 1,500 ft. The main difficulty in perforating these wells is the ~20,000 psi high-pressure environment. Under these conditions, the challenges include creating sufficiently large entrance holes in the casing, minimizing the high-risk of equipment damage due to gunshock, and reducing the amount of perforating debris created. Perforating several intervals in a single run is required to complement single-trip multizone systems. Perforating all zones simultaneously in a single trip saves time and reduces risk when compared with stacked frac-pack completions requiring multiple trips for each zone. Safety and cost reduction are extremely important in deepwater operations. Risk control is very important because gunshock and/or debris problems can lead to multimillion dollar losses in nonproductive time, and in extreme cases, gunshock problems can lead to lost wells. To undertake these challenges, a new Low Perforating Shock and Debris (LPSD) gun system was used. In comparison with standard high-pressure guns, the LPSD gun system produces much less gunshock and negligible amounts of debris; thus, minimizing gunshock risk and reducing cleanup runs typically needed to recover perforating debris. LPSD guns produce negligible amounts of debris because LPSD guns retain all the metallic components inside the guns, including the shaped charge cases, which remain virtually intact inside of the guns. A key element in planning these perforating jobs is gunshock prediction to evaluate if the equipment will be able to withstand the transient loads produced by the perforating guns. The gunshock prediction process is described in detail in this paper. For a typical 4- to 6-zone 1,500 ft gross length perforating job, the time needed from picking up the first gun to laying out the last gun averages 84 hours. All zones are simultaneously perforated, which eliminates at least three perforating runs per well, saving approximately 9.2 days per well while minimizing personnel exposure. By perforating the largest deepwater high-pressure wells in the Gulf of Mexico's Lower-Tertiary play with LPSD guns, we minimized personnel exposure, minimized debris and reduced execution time up to 72%.

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.

We present a new perforating technology based on new wireline conveyance equipment and advanced downhole modeling to maximize operational efficiency in long pay-zones under all pressure conditions. Results of perforating jobs of long pay-zones carried out on wireline in very short times compete with traditional Tubing Conveyed Perforation (TCP) operations which take much more time. Also, perforating jobs with large gun sizes that until recently were not possible in a single run with traditional wireline conveyance, are now efficiently executed in a single run. The new technology that allows conveying long lengths of perforating guns on wireline in a single run is based on four main elements: wireline systems with safe working loads up to 30,000 lbf, cutting-edge shock resistant mechanical weak points and disconnect systems, conveyance modeling, and an advanced transient dynamic modeling for perforating shock prediction. The perforating job design modeling is based on the reservoir zones and completion information, both a conveyance and a wellbore dynamics and shock simulation are carried out to determine the highest payload that can be more safely deployed per wireline run, and with the number of runs required, costs and risks are compared between wireline and TCP shoot and pull operations. For a well with a 750 ft thick pay zone, a North Sea operator requested a comparison between this new wireline perforating technology and conventional electric wireline deployment in terms of reservoir productivity, risks, and operational performance. For this well TCP was not considered due to reservoir and operational risks and challenges. Compared to the conventional electric wireline conveyance this new perforating technology offers better efficiency with only two wireline runs using a cable with 18,000 lbf of safe working load and a 10 Kpsi surface pressure control equipment compared to 6 to 8 conventional runs. The longest run consisted of 388 ft of 3 3/8″ guns, which was a new world record on wireline, with energetic liner charges and dynamic underbalance to ensure maximum perforation tunnel cleanup and well productivity. The total operational time for the perforating job was significantly less than conventional electric wireline, which translated into significant rig time savings. This paper demonstrates how the application of innovative technologies have minimized the risks of wireline conveyance with long and heavy perforating gun strings. We utilized well and reservoir information to design a more safe and reliable job execution, including prediction of perforating shock, tension profiles and wellbore dynamics. The new perforating technologies described in this paper have extended considerably the range of perforating jobs where wireline conveyance can be more efficient than traditional coiled tubing and tubing conveyed perforating.