Water issues - optimising well yield

摘要

Water is the world's most precious resource, with some 98% of the planet's fresh water lying beneath the surface, within sand and gravel formations or in the voids and fractures of bedrock Bringing it to the surface - either to develop, protect or restore - is an environmental challenge and in this issue some recent developments in the field of waterwell drilling are considered in the article on pages 4-14. One particular aspect of interest is making the most of wells which have been in place for some time, but are not producing the required yield: in this regard hydrofracturing is one of the most effective means of enhancing well productivity. Basically, it is a waterwell development/rehabilitation method used to increase the yields of low-production wells completed in rock where the fracture/joint systems are so poorly developed or so tight that little or no water can move through them. Hydrofracturing of waterwells has been used in the United States since about the late 1980s on a widespread basis. The primary application has been in 6-in waterwells of 250 to 600 ft deep (75 to 180 m). These wells, which can become plugged by natural sediments, are primarily for domestic usage with some applications in 8-in and 10-in wells for commercial, industrial and municipal uses. The most common geology found in wells that are hydrofractured, is basement igneous type rocks such as gabbros, granites, schists, etc. There are some areas where shales and slates predominate as well as others with limestone and sandstone. Most "fracturing" occurs in hard rocks of between 1,000 to 2,500 lb/in~2 and 300 to 800 lb/in~2 in softer rocks. The most common pumping flow rates are between 65 and 120 gal/mm. Some means of 'propping' to keep the fractures open, such as sand, glass beads, etc., were experimented with early on in development but now are seldom used because of their uncertain benefit, if any.