Nautilus Minerals is the world leader in exploration and development of deep ocean seafloor massive sulphide (SMS) resources. The company is currently focused on generating its resource pipeline in the Western Pacific and its first development project for recovery of high-grade copper, gold and silver mineralisation from its Solwara 1 site in the Bismarck Sea, Papua New Guinea.The Solwara 1 site is located at a water depth of 1,600 -1800 meters. Over 150 SMS seafloor surface samples have been collected from chimney structures during dives made by remote operated vehicles (ROVs).In order to obtain an understanding of the sub-surface mineralisation, Nautilus carried out 3 drilling programs between 2006 and 2008 which allowed the calculation of a Canadian NI43-101 compliant resource estimate. The initial 2006 campaign was based on conventional offshore surface driven drill equipment. In conjunction with the subsea remote technology industry, Nautilus subsequently developed an ROV operated seafloor drill system with improved drill core recovery and efficiency. This technology was developed further with a 2010 drill program to increase the available geological knowledge at Solwara 1 and adjacent prospects. This paper will provide an overview of Nautilus' SMS drilling requirements and recap the evolution of drilling technology and operations undertaken at Solwara 1 from 2006 to 2010. The paper will also discuss the potential of further developments in drilling techniques and equipment to increase or improve drilling depth, efficiency and data quality.

Summary Near-wellbore diversion of fracturing fluid and proppant is a common approach to achieving improved treatment efficiency when refracturing horizontal wells for expanding treatment coverage within the lateral. There are five broad categories of near-wellbore diversion methods: particulate diversion for bridging open fractures connected to the wellbore; perforation sealing to limit injectivity into open perforation clusters at the wellbore; filling up the drained fracture system with water for achieving more uniform pressurization (i.e., fill-up); injection rate cycling/hesitation fracturing; and mechanical isolation by installing cemented or expandable liners in the lateral followed by plug-and-perforated stimulation. These tactics can be used standalone or combined. Particulate diverting agents can be additionally categorized by particle type (e.g., granular, fibrous) and solubility characteristics. Perforation sealing agents consist of deformable and rigid/spherical subtypes, both of which can be further categorized by solubility characteristics. In this study, treatment and production data for 72 ConocoPhillips refractured wells in a North America shale play were analyzed to evaluate the effectiveness of the various near-wellbore diversion methods and materials. An index was formulated using information on reservoir depletion to normalize changes in bottomhole fracture pressure over time. This was determined by periodically discontinuing injection to obtain instantaneous shut-in pressures (ISIPs) over the course of the treatment. The calculated indices were plotted for each type of diverting system to compare trends for gaining insight on in-situ stress buildup. Production data grouped by different diversion methods were also analyzed. The near-wellbore diversion methods included mixed-size particulates with and without fibrous materials, deformable and rigid perforation sealers, fill-up tactics in which near-wellbore diverting agents were not used, and mechanical isolation by cementing a newly installed liner in the lateral followed by plug-and-perforated stimulation. Fracture hit analysis of offset well treatments indicated that refracturing treatments using particulate diverters were heel biased with respect to reservoir repressurization. The study showed that the incremental pressure as a result of diverter landing on perforations is a poor indication of diverter efficiency. A non-normalized ISIP trend is misleading as an indicator for post-refracturing well performance. Refractured wells with either particulate diverters or perforation sealers both show initial fluid fill-up into the depleted region before the stress buildup plateaus. Wells that have liners installed and cemented inside the original wellbore and that are then restimulated with standard plug-and-perforated techniques show superior performance compared with all diversion methods used in bullhead refracturing treatments. Choice of diversion can have a significant impact on results, but not all particulate diverters or perforation sealers behave similarly. Wells refractured using only the fill-up method have long-term productivity on par with or better than wells refractured with most types of diverting agents.

Summary Hydraulic-fracturing treatments in shale infill wells are often impacted by existing parent-well depletion and asymmetrical fracture growth. These phenomena can result in excessive load-water production, deposition of proppant and deformation of casing in the parent well, and unbalanced stimulation of infill wells. This study determines the effectiveness of particulate materials (i.e., far-field diverting agents) for mitigating the above negative outcomes by bridging near the extremities of dominant fracture wings. Fracture propagation was modeled to characterize the width profile at fracture extremities in a depleted-stress environment. A slotted-disk device was used to evaluate and optimize particulate blends for bridging slots representative of width near the fracture tip. Rheological tests replicating the downhole environment were used to formulate a system for transporting the diverting materials. Statistical analysis of 511 fracture hits at 30 parent wells was performed on key treatment indicators by the category of diverter type and post-hit parent-well condition. Production trends of the influenced wells were compared to area-specific type curves and offset wells without diverter trials. On the basis of the simulation and testing results, two types of high-graded far-field diverter systems were field-tested in a shale play: dissolvable, extremely fine particulate mixed with a 100-mesh sand, and mixtures of a nominal 325-mesh silica flour and a 100-mesh sand. Proppant dust collected at the fracturing site was also evaluated for replacing commercial silica flour. High-graded blends of the above diverting systems demonstrated superior fracture-hit and productivity metrics as compared to the base case of not applying far-field diverters. The silica flour and 100-mesh-sand mixture performed on a par with the significantly more expensive blend of dissolvable fine particulate and 100-mesh sand. Borate-crosslinked-guar gel was an effective carrying fluid for transporting diverting materials to the fracture extremities. Statistical analysis of fracture-hit events shows that the application of far-field diverters did not reduce the magnitude of pressure buildups during fracture hits; however, it significantly increases the post-hit pressure-falloff rate at the parent wells. On the basis of the area-specific type curves, pumping far-field diverters increased the P50 estimated ultimate recovery (EUR) by approximately 6% compared with the base cases of not applying diverters. For all the wells impacted by far-field diverters, the infill wells saw larger benefits with an increment of P50 EUR by approximately 7% compared with the parent wells.

Summary Studies have shown that achieving a consistent perforation hole size in casing (i.e., entry hole) and zero-phase perforation gun orientation led to improved treatment distribution among multiple perforation clusters in plug-and-perf limited entry treatments. In addition to reducing variation in the perforation entry hole by establishing uniformity in gun clearance and the angle of incidence of the perforation jet at the wall of the casing, oriented perforating has been shown to minimize the tendency of proppant to separate from the fracturing fluid while traveling across the perforated intervals (inertial effect) and mitigate nonuniform entry hole erosion due to gravity-induced proppant stratification. The primary goal of this study was to determine the controllable perforating gun elements and accessories that effect the accuracy of gun orientation and entry hole dimensions. Surface tests were conducted at manufacturing facilities for determining the characteristics of the entry holes in pipe produced by shaped explosive charges using various system configurations and the robustness of various gun orientation devices. Promising perforating systems were then used in wellbores to create calibration entry holes (downhole tests) that were measured for equivalent diameter and orientation accuracy using high-resolution acoustic imaging before conducting treatments. This process enabled components of the perforating system influencing entry hole size and gun orientation to be evaluated and modified, as necessary. Elements of the perforating system and downhole environment that influenced entry hole size and consistency included casing type, cement sheath characteristics, perforating gun clearance and orientation, perforating charge type and density, packing arrangement of multiple charges, charge tube and charge carrier design, gun detonation system, hydrostatic pressure, and locking devices. Achieving tight control of these elements significantly reduced variation in entry hole size. Deviations from surface and downhole testing results were commonly attributed to using perforating system elements in the field that differed from those used by the manufacturer in surface testing. Factors affecting gun orientation accuracy and consistency included weight bar type, gun string length, weight, and stiffness, the presence of modified standoff bands, progressive gun deformation during firing, wellbore tortuosity, and self-orienting devices. Several orientation systems were found that achieved orientation within the target 20°-window. To assess the value of this workflow process, the paper includes information on the results of diagnostic tests for evaluating the accuracy of the ultrasonic measuring device, the derivation process used for determining coefficients for a two-component perforation erosion model, and the use of the derived erosion rate coefficients for computing the mass of proppant that enters each perforation and perforation cluster during a fracturing treatment.

This study documents an ongoing analysis of frac plug integrity and inter-cluster treatment distribution using multiple datasets. It includes post-treatment acoustic imaging data from three Montney pads, in which the dimensions of 3538 perforations and casing wear patterns at 150 frac plug setting locations were determined. The analysis process features an iterative approach to improving execution performance during field appraisal – execute the design, measure performance, identify failures, and then implement an improved design. This approach identified execution performance issues that would have otherwise been undetected and provided insights that were used to inform manufacturers of necessary design improvements. The fiber optic and acoustic imaging programs for Pad 1 indicated loss of frac plug isolation in 70% of stages. Acoustic imaging data gathered from Pad 2 indicated loss of frac plug isolation in 57.5% of stages. Additionally, the measured diameters of eroded perforations were smaller than the expected unstimulated diameter in 48% of measurements. This finding revealed a discrepancy in the perforation-charge manufacturer's published performance information which led to unintended treatment behavior. Building on Pad 2 results, multiple vendors were engaged to provide engineered solutions to the issues identified through acoustic imaging campaigns for potential implementation on future wells. The findings from this exercise confirmed the underperformance of dissolvable frac plug technology and the importance of verifying perforation performance by conducting surface tests that are representative of field conditions. The outcome led to modified perforation charges and dissolvable frac plugs for trial on Pad 3. Outputs from the analysis performed on Pad 3 revealed improved performance, with confinement issues identified in only 28% of the total stages. Initial unstimulated perforation diameters were within 3.59% of the pre-job surface validation tests. Improvements contributed to better treatment conformance relative to Pad 1 and Pad 2.