Downhole temperature data obtained by either temperature logging or fiber-optic cables have been used to evaluate stimulation treatments and post-stimulation performance of horizontal wells with multiple fractures. Field cases qualitatively show capabilities of detecting creation of transverse fractures, poor zonal isolation, and inflow locations, although downhole temperature behavior in those wells is not fully understood from the theoretical modeling perspective.
In this study, we present comprehensive numerical flow and thermal models for a horizontal well with multiple fractures. The well experiences single-phase water flow during injection and shut-in, and gas/water two-phase flow during production. These models are formulated for reservoir and wellbore domains with consideration of their coupling. The reservoir models are formulated in 3D space using mass conservation of each component and thermal energy conservation with Darcy's law in transient conditions. The wellbore models are also transient, and formulated for 1D space using mass conservation of each component, conservation of combined-phase momentum, and total energy conservation. The wellbore- and sandface-temperature profiles are obtained as solutions of these models. These models enable us to simulate field operations in multistage-fracturing treatment; injection and shut-in occur alternately for each stage from toe to heel with zonal isolation. After the stimulation treatments, these models are used to simulate temperature behavior during production in gas/water two-phase flow.
We show an example of a single fracture in which the developed model simulates temperature behavior during injection, shut-in, and production to show capabilities of the developed model. This study shows that injected fluid makes the fluid temperature in the fracture lower than the geothermal temperature even after 1 month of shut-in. This affects the temperature interpretation during production because the initial temperature is different from the geothermal temperature assumed as the initial temperature by most studies published previously. A synthetic case with five fractures demonstrates capabilities of detection of created-fracture locations from the shut-in temperature profile. In addition, we apply the model to a field case of distributed-temperature-sensor (DTS) temperature profiles during warmback after multistage hydraulic fracturing, and 30 days after the start of the production in this well. The good match obtained between this model and the DTS data from this well indicates how this modeling approach can be used to estimate the production from individual perforation clusters. The case studies illustrate qualitative interpretations in situations occurring in fields, such as warm-up behavior with multiple clusters during the shut-in period.
This paper provides insights from the theoretical modeling perspective for downhole temperature interpretation qualitatively performed at the current time. It also discusses the validity of the assumptions made in previous studies and precautions relevant to those assumptions.