Wax Deposition Pattern in Wellbore Region of Deep Condensate Gas Reservoir and Its Prevention: A Combined Experimental and Simulation Study

Abstract

When wax deposition behavior occurs, gas condensate well suffers from moderate to serve reduction of productivity, even wellbore region blockage. For the operation and maintenance of a gas condensate well production system, a new methodology is needed to understand the wax deposition pattern in the wellbore region and assess the wax prevention under wellbore conditions. This paper establishes a phase envelope relationship in phase-behavior of typical condensate gas flow. The experiments map the potential deposition location in the wellbore region and capture the chemical wax inhibition performance in terms of wax appearance temperature (WAT), wax crystal morphology, and wax inhibiting rate, etc. The fluid component in wells for determining the envelope relationship in phase-behavior was corrected based on the gas-oil ratio of the actual gas condensate well and the carbon number distribution of the produced condensate oil-gas. The cold finger apparatus and dynamic wax inhibition measurement apparatus were designed to test wax deposition characteristics and evaluate chemical wax inhibition performance. The main test unit comprises a fully-closed high-pressure autoclave and cold finger capable of a maximum temperature of 285 °F and a maximum pressure of 16000 psi. The condensate mixtures were sampled from the wellbore region by downhole fluid sampling method. Starting from chemical wax prevention in wellbore flow, the wax crystal-improved wax inhibitor, which was mainly composed of long-chain hydrocarbons and polymers with polar groups, was employed. The temperature difference, intake pressure, stirring rate, and amount of wax inhibitor were controlled in the experiments. The wax content, WAT, and wax crystal structural characteristics of condensate systems showed noticeable differences from well to well. Using the matched component by the simulation, the wellbore temperature and pressure profiles are reliably predicted, and the envelope relationship in phase behavior of condensate gas flow is reasonably determined. Thermal and molecular diffusion are still the main mechanisms for driving wax deposition behavior in wellbore regions. The critical conditions for wax precipitation, wax deposition characteristics, and potential impact of wax deposition pattern are formulated. With the combined wellbore temperature and pressure profiles, the universal relationship schema for identifying deposition location is derived. The wax deposition location obtained from the schema agrees well with what was detected in actual production. Chemical wax prevention is an effective way to inhibit wax deposition. A maximum WAT reduction of 80% and a wax inhibiting rate of 90% could be achieved with the wax crystal improved wax inhibitor at a concentration of 0.25 wt.%. Understanding the wax deposition pattern in the wellbore region is significant for flow assurance and well operation. It provides evidence for wax prevention in wellbore flow and promotes deep condensate gas reservoir development and production efficiency.