Array

Key Takeaways

• Seismic geophone arrays use the spatial separation between elements to attenuate coherent noise with low apparent velocity while passing high-apparent-velocity reflected signal: A linear array of n geophones at d-metre spacing (each producing a signal that is summed before recording) has a spatial response function that passes waves with apparent wavelengths much larger than the array length (n times d) and attenuates waves whose apparent wavelengths fall within the array length range. Ground roll (Rayleigh surface waves) with typical apparent velocities of 150 to 600 m/s arrives at low apparent wavelengths (8 to 40 metres at 15 to 40 Hz dominant frequency), while reflected compressional waves arrive at much higher apparent velocities (2,500 to 6,000 m/s) and correspondingly larger apparent wavelengths. An array of 6 geophones at 5-metre spacing (30-metre array length) produces a null response at the 30-metre ground-roll apparent wavelength, attenuating the dominant noise by 20 to 30 dB while transmitting greater than 90 percent of the reflected-wave amplitude. In WCSB 3D seismic acquisition in the Dawson Creek area, standard receiver group arrays of 5 to 8 geophones at 3 to 5-metre group spacing (15 to 40-metre total array length) are designed from pilot shot analysis of the local ground-roll apparent wavelength and are the primary field tool for ground-roll suppression before processing.
• LWD resistivity array tools provide simultaneous measurements at multiple depths of investigation by using transmitter-receiver spacings from 16 to 72 inches to resolve the invasion profile around the borehole and determine true formation resistivity: Array propagation resistivity (APR) tools use multiple transmitter-receiver pairs at different axial spacings along the drill collar, each measuring formation resistivity at a different depth of radial investigation (from approximately 25 cm for the shortest spacing to 150 cm for the longest spacing). By comparing the resistivity readings at each depth of investigation, the formation evaluation engineer constructs an invasion profile that shows whether the formation resistivity decreases from the borehole outward (indicating a saline mud filtrate has invaded a resistive hydrocarbon zone) or increases outward (indicating a freshwater filtrate has invaded a conductive water zone). The shallowest-reading curve represents the flushed zone (Rxo), the deepest-reading curve represents the uninvaded formation resistivity (Rt), and the intermediate curves constrain the invasion diameter. This invasion profile information is critical for applying the Archie Equation correctly: if Rt is read from a curve that has not penetrated beyond the invasion front, the computed Sw will be that of the invaded zone rather than the undisturbed reservoir, systematically overestimating or underestimating water saturation depending on the filtrate and formation water resistivities.
• FMI Formation MicroImager arrays of 192 button electrodes provide borehole wall images at 5-mm resolution that enable structural dip, fracture, and sedimentary feature characterisation at centimetre scale: The FMI tool (Schlumberger) arrays 192 small button electrodes in four rows of 48 pads (each pad covering approximately 22 degrees of the borehole circumference) that rotate and maintain contact with the borehole wall during logging. Each electrode injects a small current into the formation and measures the return current, which is proportional to the formation's microresistivity at the button location; high-conductivity (high current return) features such as clay-filled vugs and clay laminations appear dark on the processed image, while resistive features (fractures filled with resistite cement, high-porosity gas-filled zones) appear light. The spatial resolution of the array (5-mm button diameter at 5-mm spacing) enables identification of laminated sands at centimetre scale in the Cardium and Viking formations, individual fractures (both natural and drilling-induced) in Duvernay and Nisku carbonates, and cross-bedding dips in fluvial Mannville sands. The structural dip of bedding planes is identified from the sinusoidal trace that a dipping plane makes on the unrolled borehole image, and the dip magnitude and azimuth are computed from the depth difference between the top and bottom of the sinusoid and the sensor orientation data from the tool's accelerometer and magnetometer package.
• Distributed acoustic sensing (DAS) arrays use Rayleigh backscatter from a fibre optic cable to provide continuous, spatially distributed acoustic measurements at every metre along the cable simultaneously: DAS arrays function by sending laser pulses down an optical fibre and detecting the backscattered light from natural imperfections (Rayleigh scatterers) in the silica glass. When acoustic waves (including seismic waves, hydraulic fracture-induced microseismicity, or fluid-flow noise in the wellbore) cause strain in the fibre, the phase of the backscattered light changes in proportion to the strain. By correlating the backscattered signals from successive pulses at each distance along the fibre (using a technique called coherent optical time-domain reflectometry, C-OTDR), the DAS interrogator reconstructs the acoustic waveform at every 1-metre sensing point along the fibre, providing a continuous spatial array of thousands of sensing elements without any downhole electronics. In WCSB Montney completions, DAS fibres cemented behind casing or deployed in an offset monitoring well during multistage hydraulic fracture operations provide real-time acoustic images of fracture initiation, propagation, and inter-well communication at a spatial resolution of 1 metre, equivalent to the capabilities of a dense seismometer array deployed continuously in the wellbore at no incremental capital cost per sensor after the initial fibre installation.
• Perforation gun arrays are designed to maximise the area open to flow and create the optimal hydraulic fracture initiation geometry for a specific formation stress state and completion strategy: A perforation gun array consists of multiple shaped charges arranged at defined azimuths and spacings along a carrier that is positioned in the wellbore for a single firing event. The charges are typically arranged in one of several phasing patterns: 60-degree phasing (six charges equally distributed around 360 degrees at 60-degree separation per 0.25-metre interval, producing 24 perforations per metre), 90-degree phasing (four charges at 90-degree separation), or single-plane arrays (all charges fired in the same azimuth plane, as used in oriented perforating). The array geometry determines both the total area open to flow (number of perforations times the cross-sectional area per perforation tunnel) and the spatial distribution of flow entry points relative to the hydraulic fracture preferred plane. Oriented perforation arrays fire all charges in the direction of maximum horizontal stress (sigma_H, perpendicular to minimum horizontal stress sigma_h), eliminating the near-wellbore tortuosity that occurs when perforations not aligned with the fracture plane are connected to the fracture network by a curved near-wellbore path. In the Duvernay Formation at Kaybob South, oriented perforation arrays at 10 to 12 perforations per cluster in the sigma_H azimuth reduce fracture initiation pressure by 5 to 8 MPa compared to 360-degree phased arrays, allowing fracture initiation at lower pump pressure and reducing surface equipment rating requirements for the high-pressure completion pumps.