Cleaning Up Water Blocking in Gas Reservoirs by Microwave Heating
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 101072, "Clean Up Water Blocking in Gas Reservoirs by Microwave Heating: Laboratory Studies," by Gao Li, Yingfeng Meng, SPE, and Hongming Tang, Southwest Petroleum Inst., China, prepared for the 2006 SPE International Oil & Gas Conference and Exhibition in China, Beijing, 5–7 December.
Water blocking caused by fluid invasion during drilling, completion, and stimulation has been suspected of reducing or blocking gas-well deliverability. A technique that uses microwave heating to clean up water blocking in sandstone gas reservoirs is presented. Water-block cleanup by use of microwave heating is encouraging for field application, but further laboratory studies are required for the next phase of this project.

Introduction
Liquid will invade gas reservoirs when water-based fluids are used for drilling, completion, and stimulation. This water invasion will decrease the permeability, reducing flow capability or causing water blocking. Water blocking may lead to underestimating the gas-well productivity and reservoir value and to applying the wrong development strategy. Water blocking is a type of formation damage that restricts development of low-permeability gas reservoirs.
Water blocking can be avoided or alleviated to some degree by use of underbalanced drilling, nonwetting-phase- and gas-based working fluids, and surface-tension-reduction techniques. After water blocking has happened, bypassing the damaged area may reduce the degree of damage (e.g., deep-penetration perforation, fracturing, and acid fracturing). Decreasing the water saturation of damaged areas (increasing drawdown, dry-gas injection, or extending shut-in time) or decreasing surface tension also may be effective. However, these methods cannot relieve water-block damage completely.
Formation-heating technology (FHT) can improve the crude-oil fluidity, dehydrate clay, and enhance near-wellbore formation permeability and can be used to remedy water blocking by injecting hot gas to heat the formation near the wellbore. FHT has been tested and applied in oilfield production. However, FHT uses heat conduction and convection in the rock and gas to heat the formation. Because the heat capacity of gas is limited and heat conduction through rock is slow, the heating process is slow and inefficient.
Microwave heating can heat materials rapidly and efficiently. Microwave heating is used to increase fluidity of heavy oil and to produce gas hydrate. This paper studied the influence of microwave heating on sandstone gas reservoirs and the feasibility of clearing water blockage.

Basic Microwave Heating
Microwaves are extremely high-frequency electromagnetic waves with a wavelength between 1 mm and 1 m and frequency from 300 GHz to 300 MHz, respectively. Used on dielectric materials, microwaves can cause electronic polarization or atomic polarization, interfacial polarization, and dipolarity turning to polarization. The polarization of dielectric materials can cause power dissipation, which raises the material's temperature. Conventional heating depends on heat conduction, convection, and radiation with slow heating from external to internal, while microwave heating can heat both the internal and the external at the same time with high heating speed and efficiency. Microwave energy can trans-form into heat energy instantaneously.