Radius of Investigation

Key Takeaways

• The diagnostic log-log plot of pressure change and pressure derivative versus time (the standard well test interpretation diagnostic) directly reflects the reservoir features within the radius of investigation at each time on the x-axis: the early-time data (small radii of investigation) reflects wellbore storage effects and near-wellbore damage (skin) at radial distances of a few feet; the middle-time data during infinite-acting radial flow (when the investigation has propagated past the damaged zone but not yet reached any boundary) reflects the average reservoir permeability and fluid mobility within a few hundred to several thousand feet of the wellbore, depending on the test duration; the late-time data reflects reservoir boundaries, faults, permeability barriers, and natural fracture networks at distances that may extend to the drainage radius of the well; the sequence of flow regimes visible on the log-log diagnostic (wellbore storage, damaged zone, radial flow, linear flow, boundary effects) is a temporal record of the pressure investigation front encountering successively larger-scale features as the radius of investigation expands with time.
• The design of a pressure buildup test to achieve a specific investigative objective (measuring permeability to a specific depth, detecting a known fault at a measured distance, confirming communication with an adjacent well) requires calculating the minimum shut-in time needed for the radius of investigation to reach the target distance: for a reservoir with known permeability, porosity, fluid viscosity, and total compressibility, the time required to reach a radius of r feet can be calculated by rearranging the radius of investigation equation to solve for t; in a tight formation with low permeability (0.01 md), the radius of investigation grows slowly and a 72-hour buildup test may characterize only 500-1,000 feet of reservoir, leaving faults and permeability baffles at greater distances undetected; in a high-permeability formation (100 md), the same 72-hour test may investigate 5,000-10,000 feet, potentially reaching regional boundaries; this permeability dependence of the investigation rate is why well test design requires knowledge of expected reservoir quality, and why buildup tests in tight formations must be much longer than those in high-permeability reservoirs to achieve the same investigative depth.
• The concept of radius of investigation must be distinguished from the radius of drainage (the distance from the well from which reservoir fluid is actually being produced at economic rates at any point in time), which grows much more slowly than the investigation radius; the investigation radius grows approximately as the square root of time, reaching thousands of feet within days for moderate-permeability formations; the drainage radius grows on reservoir-depletion timescales (months to years), as the low-pressure cone around the well gradually expands through the reservoir as fluid is produced; a well test conducted after only a few months of production may have an investigation radius that has already reached the well's ultimate drainage boundary (revealing the boundary on the derivative log), while the actual drainage front (the zone from which significant fluid depletion has occurred) may still be within a few hundred feet of the wellbore; misinterpreting the radius of investigation as the drainage radius is a common conceptual error that leads to incorrect estimates of the reservoir volume that has been effectively drained and the remaining undrained volume available for future production.
• Interference testing between wells uses the radius of investigation concept to determine the communication efficiency between adjacent wells: when a rate change is imposed at a source well, the pressure response at an observation well located at a known distance provides direct measurement of the reservoir transmissibility and storativity in the inter-well region; the time at which the pressure response becomes detectable at the observation well corresponds to the time at which the radius of investigation from the source well reaches the observation well distance; wells with high transmissibility between them show interference response rapidly (large radius of investigation at any time), while low transmissibility or a permeability barrier between the wells delays or eliminates the interference signal; modern permanent downhole gauges that record pressure continuously have made interference testing much more practical by providing high-quality pressure records at both the source and observation wells without requiring a dedicated well intervention, enabling reservoir connectivity mapping that guides infill drilling and injection pattern optimization in mature fields.
• The radius of investigation in hydraulically fractured wells is complicated by the non-radial geometry of the fracture and the multiple flow regimes that occur before the classical radial flow regime develops: during early production from a hydraulically fractured well, flow is predominantly linear (into and along the hydraulic fracture), and the "investigation" propagates linearly into the matrix perpendicular to the fracture face rather than radially from the wellbore; a classical radius of investigation concept (radial propagation from a point source) does not apply during linear or bilinear flow, and the effective investigation depth in the fracture direction is much greater (thousands of feet into the fracture) while the investigation depth perpendicular to the fracture may be limited to tens of feet of matrix drainage depending on matrix permeability; the transition from linear to pseudo-radial flow occurs when the investigation in the fracture-perpendicular direction has sampled enough matrix volume for the composite well-fracture system to behave approximately like a radial source, and this transition may take months to years in tight matrix formations, making the simple radius of investigation concept applicable only at very late times in unconventional well production analysis.