How is it pro­duced?

Down to Earth - - COVER STORY -

AF­TER A patch of land is cleared, a hole, some 2000-3000 m deep, is dug ver­ti­cally till it reaches the shale rocks. Steel tubes called cas­ings are in­serted and ce­mented in the hole to isolate the well from the sur­round­ing rocks and aquifers. Now hor­i­zon­tal holes rang­ing from 1,000-3,000 m are dug to ac­cess the rocks at var­i­ous points. Next step is to frac­ture the rocks to free the gas trapped within. For this wa­ter mixed w with sand and chem­i­cals is pumped into t the ground at high pres­sure to force it t through the per­fo­ra­tions. This cre­ates c cracks in un­der­ground rocks. Hy­draulic f frac­tur­ing of rocks is also called frack­ing. S Sand holds the cracks open, al­low­ing the g gas to flow into the bore well.

Given the length of hor­i­zon­tal wells, h hy­draulic frac­tur­ing is of­ten con­ducted in 10-20 stages, where each stage fo fo­cuses on a limited lin­ear sec­tion and m may be re­peated nu­mer­ous times. This multi-stage frac­tur­ing re­quires large vol­umes of wa­ter. A stan­dard sin­glestage hy­draulic frac­tur­ing may pump down sev­eral hun­dred cu­bic me­tres of wa­ter to­gether with prop­pant and a mix­ture of chem­i­cal ad­di­tives. In mul­ti­stages the to­tal vol­ume of wa­ter used might reach up to 20,000 cu­bic me­tres per well and the amount of prop­pant up to 4,000 tonnes.

Some of the frac­tur­ing fluid in­jected into the well will re­turn to the sur­face along with wa­ter that oc­curs nat­u­rally in the rocks. This is brought to the sur­face, col­lected, treated and reused or dis­posed of. Along with this, nat­u­ral gas is also re­leased, which is col­lected and treated.

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