Degasification of the Mary Lee coalbed near Oak Grove, Jefferson County, Alabama, by vertical borehole in advance of mining (1974)

A 5 h o l e p a t t e r n was d r i l l e d from t h e s u r f a c e a t a s i t e n e a r Oak Grove, J e f f e r s o n County, A l a . , t o d e g a s i f y t h e g a s s y Mary Lee coalbed i n advance o f mining. Gas and w a t e r p r o d u c t i o n was moni tored f o r 1 y e a r , 4 months t o a l low gas f low t o s t a b i l i z e f u l l y . A th ickened w a t e r s t i m u l a t i o n t rea tment was des igned and a p p l i e d i n one h o l e o f t h e p a t t e r n t o e v a l u a t e t h i s d e g a s i f i c a t i o n t echn ique f o r t h e Mary Lee coa lbed . A gas p r o d u c t i o n r a t e o f 70,000 f t 3 /day a f t e r s t i m u l a t i o n t r e a t m e n t i n d i c a t e s t h a t t h e u s e of v e r t i c a l boreho les coupled w i t h h y d r a u l i c f r a c t u r i n g i n advance o f mining t h e Mary Lee coa lbed would p r o v i d e f o r more r a p i d d e g a s i f i c a t i o n and could p r o v i d e a p o s s i b l e s o u r c e o f n a t u r a l gas . INTRODUCTION D e g a s i f i c a t i o n o f coa lbeds through v e r t i c a l boreho les from t h e s u r f a c e i s p a r t o f t h e comprehensive methane c o n t r o l r e s e a r c h program conducted by t h e Bureau o f Mines. The t echn ique h a s been a p p l i e d e x p e r i m e n t a l l y a t s e l e c t e d s i t e s i n most o f t h e major bituminous c o a l producing a r e a s i n t h e United S t a t e s . One such s i t e i s J e f f e r s o n County, A l a . , where a mine i s being developed i n t h e Mary Lee coa lbed . A d e g a s i f i c a t i o n p a t t e r n o f 5 h o l e s was d r i l l e d a t t h i s s i t e May 22 through J u l y 11, 1971. Moni tor ing o f g a s and w a t e r p r o d u c t i o n began September 16, 1971. P roduc t ion from boreho le No. 3SW was n e a r l y cont inuous u n t i l June 28, 1973, when i t was s t i m u l a t e d by h y d r a u l i c f r a c t u r e t o i n c r e a s e t h e r a t e o f d e g a s i f i c a t i o n o f t h e coa lbed . This s t u d y showed t h a t t h e combined p rocedure shou ld d e c r e a s e t h e hazard o f methane i g n i t i o n s and e x p l o s i o n s d u r i n g mining. I t a l s o demonstra ted t h e f e a s i b i l i t y o f p r o v i d i n g supplemental s o u r c e s o f f u e l through t h e recovery o f methane t h a t would o t h e r w i s e be wasted t o t h e atmosphere. G e o l o g i s t . 2 Superv i sory g e o l o g i s t . ACKNOWLEDGMENTS The au tho r s thank J . K , Weed, J . L. Huey, E . J . F i l e s , and H. E . Kerley f o r p rov id ing maps, co re d a t a , and a s s i s t a n c e i n t h e f i e l d . The au tho r s e s p e c i a l l y thank Kenneth 0 . Aust in of Calvert-Western Explora t ion Co., and H . E . Kerley, f o r review o f t h e manuscr ipt . BACKGROUND During 1971, underground coa l mines i n t h e United S t a t e s vented n e a r l y 300 m i l l i o n f t 3 o f methane t o t h e atmosphere each day. To d i l u t e t h i s gas t o s a f e l e v e l s i n t h e mines, more than 100 times t h i s q u a n t i t y o f a i r had t o be c i r c u l a t e d through t h e mine workings. I f some of t h i s gas could have been removed from t h e coalbeds before mining s t a r t e d , mine v e n t i l a t i o n c o s t s could have been reduced, and a marketable energy product could have been conserved. Commercial q u a n t i t i e s of gas have been produced from t h e P i t t sbu rgh coalbed i n Wetz-el County, W. Va. (1-2)3 where produc t ion d e c l i n e curve f o r a group of 20 w e l l s on 1,500-foot spac ing was t y p i c a l and i n d i c a t e d no p a r t i c u l a r flow c h a r a c t e r i s t i c s a s a coal-gas r e s e r v o i r . Research on d e g a s i f i c a t i o n of t h e P i t t s b u r g h coalbed by s u r f a c e boreholes t o reduce gas emissions i n t o mine workings was p ioneered by Spind le r and Poundstone (13-14). The i r r e s u l t s , coupled w i th known commercial p roduc t ion , i n d i c a t e d t h a t d e g a s i f i c a t i o n by v e r t i c a l boreholes i n advance of mining was encouraging bu t r equ i r ed f u r t h e r s t udy i n o t h e r coalbeds and us ing s t i m u l a t i o n techniques . I n 1971, a p r o j e c t was i n i t i a t e d t o exp lo re t h i s d e g a s i f i c a t i o n technique a t t e s t s i t e s l oca t ed i n t h e major coa l producing bas in s throughout t h e United S t a t e s t h a t a r e known t o have h igh ly gassy coa lbeds . The o b j e c t i v e s were t o d r i l l 5-hole p a t t e r n s i n t o s e l e c t e d gassy coa lbeds , develop r e s e r v o i r parameters , and e v a l u a t e w e l l s t i m u l a t i o n techniques f o r a c c e l e r a t i n g t h i s d e g a s i f i c a t i o n procedure. Several c r i t e r i a were used t o e s t a b l i s h t e s t s i t e s : A h i s t o r y of gas emission i n t o t h e mine workings, a s i t e l o c a t i o n i n v i r g i n coa l 3 t o 5 yea r s i n advance of mine development, a s i t e remote from o l d mine workings, and a t l e a s t one h o l e i n each p a t t e r n t o be s t imu la t ed . DRILLING AND COMPLETION The t e s t s i t e s e l e c t e d i n t h e Mary Lee coalbed was i n 23, T 18 S, R 6 W , n ea r Oak Grove, J e f f e r s o n County, A la . , i n t h e Warrior c o a l f i e l d ( f i g s . 1 -3) . The s i t e i s on a s t r u c t u r a l nose on t h e f l a n k o f t h e Sequatchie a n t i c l i n e ( f i g . 4 ) . The t e s t ho l e s were d r i l l e d i n v i r g i n c o a l i n an a r e a not be mined f o r 3 t o 5 yea r s and remote from any o l d mine. A w e l l spac ing of 1 ,500 f e e t between ho les was s e l e c t e d between t h e corner ho les of a square p a t t e r n ; a f i f t h h o l e was l oca t ed a t t h e c e n t e r of t h e square . D r i l l i n g of t he 5-hole t e s t p a t t e r n began May 22, 1971, and was completed J u l y 11, 1971. Nine-inch-diameter ho les were d r i l l e d w i t h s t anda rd r o t a r y 3Underlined numbers i n paren theses r e f e r t o i tems i n t h e l i s t of r e f e r ences preceding t h e appendixes. I wiley I dome Sequatchie anticline 7 Warrior syncline // Blue creek ant syncl ine icline W a r r i o r f i e l d LEGEND 4 Anticline Syncline Dome 0 20 40 Scale, miles d r i l l i n g equipment. D r i l l c u t t i n g s were removed by c i r c u l a t i n g a i r o r water dur ing d r i l l i n g . The ho le s were d r i l l e d t o t h e top of t he coalbed and cased t o provide f o r open ho le completion i n the coa l i n t e r v a l . Open hole completion provides t h e maximum exposure of t h e coalbed t o product ion of gas wi th minimal chance of formation plugging by invas ion of wel l cement. When d r i l l i n g t o t h e top of the coalbed was completed, the ho l e s were logged with geophysical logging equipment ( f i g . 5 ) . Gamma ray , neu t ron , d e n s i t y , and c a l i p e r logs a r e used t o determine ho l e condi t ion before cas ing the ho l e and t o eva lua t e porous zones i n t h e s t r a t a over ly ing t h e coalbed t h a t may con ta in water o r gas . The ho le s were cased with 7-inch-OD s t e e l cas ing cemented from bottom t o s u r f ace w i th expanding type cement. Seven-inch-OD cas ing was s e l e c t e d so t h a t t h e d e g a s i f i c a t i o n boreholes FIGURE 1. Map showing maior structure axes in the could be used f o r o t h e r Warrior coalfield of Alabama. purposes a f t e r mine development has bypassed them, such a s power and communication system e n t r y i n t o t h e mine, bulk supply p o i n t f o r rock d u s t , and f o r gob d e g a s i f i c a t i o n of longwall pane ls o r a f t e r p i l l a r recovery. A f t e r cas ing was s e t , t h e coalbed was cored c u t t i n g a 6inch ho le and r e t r i e v i n g a 4inch core . Coring was done wi th a i r t o l i m i t p o s s i b l e phys i ca l damage t o t h e coalbed t h a t would i n h i b i t t h e flow of gas during t h e degas i f i ca t i o n pe r iod . Immediately fol lowing co r ing , the coalbed was t e s t e d f o r formation p r e s s u r e and flow r a t e s of water and gas . For t h i s purpose a test t o o l and ?ackers on the bottom o f a s t r i n g of d r i l l p ipe were i n s e r t e d i n t o t h e h o l e , B lo un t C ou nt y S ec tio n