Artificial Lift Optimization for Shallow Carbonate Magwa and Ostracod Formations with Massive Propped Fractures

Objectives/Scope: The Magwa and Ostracod formations are tight and highly fractured carbonate reservoirs. At shallow depth (1600-1800 ft) and low stresses, wide, long and conductive propped fracture has proven to be the most effective stimulation technique for production enhancement. However, optimizing flow of the medium viscosity oil (17-27 API gravity) was a challenge both at initial phase (fracture fluid recovery and proppant flowback risks) and long-term (depletion, increasing water cut, emulsion tendency). Methods, Procedures, Process: Historically, due to shallow depth, low reservoir pressure and low GOR, the optimum artificial lift method for the wells completed in the Magwa and Ostracod reservoirs was always sucker-rod pumps (SRP) with more than 300 wells completed to date. In 2019 a pilot re-development project was initiated to unlock reservoir potential and enhance productivity by introducing a massive high-volume propped fracturing stimulation that increased production rates by several folds. Consequently, initial production rates and drawdown had to be modelled to ensure proppant pack stability. Long-term artificial lift (AL) design was optimized using developed workflow based on reservoir modelling, available post-fracturing well testing data and production history match. Results, Observations, Conclusions: Initial production results, in 16 vertical and slanted wells, were encouraging with an average 90 days production 4 to 8 times higher than of existing wells. However, the initial high gas volume and pressure is not favourable for SRP. In order to manage this, flexible AL approach was taken. Gas lift was preferred in the beginning and once the production falls below pre-defined PI and GOR, a conversion to SRP was done. Gas lift proved advantageous in handling solids such as residual proppant and in making sure that the well is free of solids before installing the pump. Continuous gas lift regime adjustments were taken to maximize drawdown. Periodical FBHP surveys were performed to calibrate the single well model for nodal analysis. However, there limitations were present in terms of maximizing the drawdown on one side and the high potential of forming GL induced emulsion on the other side. Horizontal wells with multi-stage fracturing are common field development method for such tight formations. However, in geological conditions of shallow and low temperature environment it represented a significant challenge to achieve fast and sufficient fracture fluid recovery by volume from multiple fractures without deteriorating the proppant pack stability. This paper outlines local solutions and a tailored workflow that were taken to optimize the production performance and give the brown field a second chance. Novel/Additive Information: Overcoming the different production challenges through AL is one of the keys to unlock the reservoir potential for full field re-development. The Magwa and Ostracod formations are unique for stimulation applications for shallow depth and range of reservoirs and fracture related uncertainties. An agile and flexible approach to AL allowed achieving the full technical potential of the wells and converted the project to a field development phase. The lessons learnt and resulting workflow demonstrate significant value in growing AL projects in tight and shallow formations globally.

Estimation of cell-free fetal DNA fraction from maternal plasma based on linkage disequilibrium information

Cell-free fetal DNA fraction (FF) in maternal plasma is a key parameter affecting the performance of noninvasive prenatal testing (NIPT). Accurate quantitation of FF plays a pivotal role in these tests. However, there are few methods that could determine FF with high accuracy using shallow‐depth whole‐genome sequencing data. In this study, we hypothesized that the actual FF in maternal plasma should be proportional to the discrepancy rate between the observed genotypes and inferred genotypes based on the linkage disequilibrium rule in certain polymorphism sites. Based on this hypothesis, we developed a method named Linkage Disequilibrium information-based cell-free Fetal DNA Fraction (LDFF) to accurately quantify FF in maternal plasma. This method achieves a high performance and outperforms existing methods in the fetal DNA fraction estimation. As LDFF is a gender-independent method and developed on shallow-depth samples, it can be easily incorporated into routine NIPT test and may enhance the current NIPT performance.

Preliminary Results of Regional P and S Wave Tomography at Seram, Indonesia

The Seram Trough is located in the northern part of Indonesia and has a complex tectonic setting. The uniqueness of these regions lies in the U-shape subduction system. Several models have been proposed in this region, such as one subduction system that has been rotated 90° or 180°, two subduction systems, and one subduction that having a slab roll-back that causes extension systems. In this study, we try to invert velocity and seismicity using double-difference tomography with the target of better imaging the sub-surface structure in the region. We use data catalogue collection from the Indonesian Agency of Meteorology, Climatology, and Geophysics. The length of data is 4 years from January 2015 to December 2019 from 16 permanent stations. Earthquake relocations show a focused hypocenter distribution at shallow depth, and we interpreted some of these shallow depth events are related to the magmatic activity. Event distribution also displays a steep angle of seismicity pattern that represents the dipping subduction slab. Inverted Tomography models show a band of faster velocity models that dip from North to South, suggesting a subductions slab. We also observe a possibility of a tear in the slab from the seismicity pattern and tomogram model. The slower velocity perturbation is seen at shallow depth that may associate with magmatic and frequent shallow seismicity. A possibility of partial melting is also seen with low-velocity zone at a depth of 70 km next to the fast dipping velocity.

Initial Result of P Wave Tomography Model in Sunda-Banda Arc Transition using FMTOMO

Our study area is located near island Sumbawa, Sumba, Flores, West Timor, Indonesia and East Timor, popularly known as Sunda-Banda arc transition zone. The tectonic setting is mainly controlled by the movement of the oceanic lithosphere Indo-Australian plate subducting the Eurasian plate and Northward migration of Australian continental lithosphere into western Banda-arc in the region of Flores, Sumba and Timor island. We tried to image velocity structure beneath these regions using regional events and tomography inversion model. We collected 5 years of regional events from the Indonesian Agency of Meteorology, Climatology and Geophysics. In total, we reserved 3186 events recorded on 29 stations. For data processing, we used fast marching method as ray tracing between sources and receiver. We then employed subspace inversion as the tomography procedure to estimate the best velocity model representing the tectonic model in the region. Hypocenter data distribution is concentrated on shallow parts of the region and along the Benioff zone down to a maximum depth of 400 km. One of challenge of this study is that although events are abundance, the stations used are mostly located onshore and does not extend in the south-north direction that leads us to under determined problem in the inversion process. However, checker-board models show most our target area can be retrieved to its initial model with sign of smearing effects shown start from a depth of 50 km. After six iteration and optimized selection of damping and smoothing parameters, we observed low velocity anomaly under Bali, Lombok, Sumba, East Nusa Tenggara at shallow depth that may be related with volcanic activity. Deeper low anomaly can also be seen that may be related with partial melting process. A band of fast velocity is clearly seen that goes deepen to the north depicting subducting slabs own to a depth of 300 km. We also observed a possible of fast velocity in the northern part of our stations at shallow depth that we believe may represent the back arc thrust.

Fault Rock Property Prediction On Jurassic Clastics Of The Barents Sea/Norway

We analysed the fault rocks of a compartmentalized field in the Barents Sea, in an area with several tectonic elements, which formed at different tectonic events. Standard Fault Seal Analysis (FSA) was conducted to predict the shale content of the fault rock (SGR). A static cellular model based on well data, seismic data and geological concepts served as input. The fault rock calibration workflow required various data acquired by different methods. We analysed the Mid-Triassic to Upper Jurassic clastic deposits to reconstruct the tectonic history. Apatite fission track and (U-Th)/He thermochronology were used to determine the maximum burial depths and exhumation history. The results of high-resolution shale ductility analysis (BIB-SEM), a compaction trend study, kinematic analysis and structural modelling (section balancing) served as additional input constraints for fault rock calibration. The evaluation of the results helped to reconstruct the following tectonic evolution: The orthogonal faults of the analysed area developed at an early stage, during Late Triassic to Early Jurassic times at relatively shallow depth, below 1000 m. Ongoing subsidence created accommodation space for Upper Jurassic to Cenozoic deposits with a maximum burial depth of 2000 m for the analysed Mid-Jurassic rocks. Exhumation of the area started around 10 Ma and continued through to Quaternary times. The calculated values for fault rock permeability show a wide range when using poorly constrained input for fault rock calibration: 10 to 1000 mD for SGR values around 0.08 at reservoir/reservoir juxtaposition. Fault rock calibration using above described results concluded in reliable values for fault rock permeability and ultimately, for transmissibility multipliers. The reason for the sensitivity of the fault rock calibration is a combination of multiple factors: highly permeable reservoir sandstone, shallow depth of initial faulting, maximum burial depth and low shale content at the primary reservoir level. The study shows that an accurate reconstruction of the geohistory provides essential parameters for fault rock calibration and fault rock permeability calculation. The range of values can widely scatter if preconditions are not acknowledged. Well-constrained fault rock calibration reduces the uncertainty on possible flow scenarios, increases the reliability on production forecasts and helps to determine the most efficient drainage strategy.

Effect of Fixing Earthquake Depth in ShakeAlert Algorithms on Performance for Intraslab Earthquakes

We investigate whether assuming a fixed shallow depth in the ShakeAlert network-based earthquake early warning system is sufficient to produce accurate ground-motion based alerts for intraslab earthquakes. ShakeAlert currently uses a fixed focal depth of 8 km to estimate earthquake location and magnitude. This is an appropriate way to reduce computational costs without compromising alert accuracy in California, where earthquakes typically occur on shallow crustal faults. In the Pacific Northwest (PNW), however, the most common moderate-magnitude events occur within the subducting Juan de Fuca slab at depths between ∼35 and 65 km. Using a dataset of seismic recordings from 37 Mw 4.5+ intraslab earthquakes from the PNW and Chile, we replay events through the Earthquake Point-Source Integrated Code and eqInfo2GM algorithms to estimate source parameters and compute modified Mercalli intensity (MMI) alert threshold contours. Each event is replayed twice—once using a fixed 8 km depth and a second time using the actual catalog earthquake depth. For each depth scenario, we analyze MMI III and IV contours using various performance metrics to determine the number of correctly alerted sites and measure warning times. We determine that shallow depth replays are more likely to produce errors in location estimates of greater than 50 km if the event is located outside of a seismic network. When located within a seismic network, shallow and catalog depth replays have similar epicenter estimates. Results show that applying catalog earthquake depth does not improve the accuracy of magnitude estimates or MMI alert threshold contours, or increase warning times. We conclude that using a fixed shallow earthquake depth for intraslab earthquakes will not significantly impact alert accuracy in the PNW.

Extended Reach Drilling with Coiled Tubing: A Case Study on the Alaskan North Slope That Proves the Benefits of Drilling a Straight Hole

Summary A set of five wells were to be drilled with directional coiled tubing drilling (CTD) on the North Slope of Alaska. The particular challenges of these wells were the fact that the desired laterals were targeted to be at least 6,000 ft long, at a shallow depth, almost twice the length of laterals that are regularly drilled at deeper depths. The shallow depth meant that two of the five wells involved a casing exit through three casings, which had never been attempted before. After drilling, the wells were completed with a slotted liner, run on coiled tubing (CT). This required a very smooth and straight wellbore so that the liner could be run as far as the lateral had been drilled. In this paper, we focus on one of the two wells on which triple casing exit was performed. However, the same considerations and results apply to the other wells on which the same technology has been used. Various methods were considered to increase lateral reach, including running an extended reach tool, using a friction reducer, increasing the CT size, and using a drilling bottomhole assembly (BHA) that could drill a very straight well path. All of these options were modeled with tubing forces software, and their relative effectiveness was evaluated. The drilling field results easily exceeded the minimum requirements for success. This project demonstrated record-breaking lateral lengths, a record length of liner run on CT in a single run, and a triple casing exit. The data gained from this project can be used to fine-tune the modeling for future work of a similar nature.