"Steam-Assisted Gravity Drainage: Concept, Development, Performance and Future" R.M. Butler

"STEAM-ASSISTED GRAVITY DRAINAGE: CONCEPT,
DEVELOPMENT, PERFORMANCE AND FUTURE"
R.M. BUTLER
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JCPT94-02-05 Steam-assisted Gravity Drainage: Concept, Development, Performance and Future R.M. BUTLER University of Calgary Abstract In the Steam-assisted Gravity Drainage (SAGD) process, heated oil drains from around growing steam chambers, driven by gravity, to lower horizontal wells. As the oil drains, the steam chamber advances into the reservoir. The process has several features: - The displacement of the oil is systematic and high recoveries can be obtained. - In suitable applications, oil to steam ratios higher
than those found for conventional steamflooding can be achieved. The process can be used in even the heaviest of bitumen reservoirs without extensive preheating. The feature which makes this possible is that once the oil is heated, it remains hot as it drains to the production well; this is unlike conventional steamflooding where oil which is displaced from the steam chamber tends to cool on its way to production. Steam-assisted Gravity Drainage allows steamflooding at economic rates without the bypass of steam, It
gives high recoveries in both bitumen and heavy oil reservoirs. It has been demonstrated in Canadian field trials with results which are in reasonable agreement with prior theoretical and scaled model studies. Introduction Although the steam-assisted gravity drainage (SAGD) process has application in the recovery of conventional heavy oils. it was originally conceived for the recovery of bitumen where the in SitLi viscosity is so high that conventional production methods are impractica](1,2). The quantity of bitumen in place in Canadian reservoirs is as large as that for
conventional crude oil in the Middle EaSt(3). Thus, the challenge of developing suitable methods for recovery is of very great practical importance. Iii the niain Canadian reservoirs - Athabasca, Cold Lake and Peace River, the bitumen is essentially immobile and injection of fluids is usually very difficult. In SitLi recovery methods generally depend Llpoii heatina the bitunien iii t, order to' reduce its viscosity@ both hezitiiig with steam and in s itu combustion have been used. One of the main problems encountered is that, even if the bitumen is heated, it cannot be pushed
through cold reservoir without regaining its lost viscosity@ this prevents adequate flow. It is necessary to keep the bitumen hot as it flows to the production well. One scheme for doing this which has becti studied is reverse combustion(4). In this approach, the tai- sand is ignited near the production well and the flame front inoves @tuainst the flow of the 44 _ conihustic@n air which is injected into another well. A major problem is obtaining adequate air injectivity. Another problem, and an even larg(@r one, is that secondary combustion fronts tend to occur near
the injection well and this has caused the process to be unsuccesSfUI(5). A bettl,-r approach is that of cyclic steam stimulation. In this method, steam is first injected into the reservoir through the production ,ell: in bitumen reservoirs, it is frequently necessary to use injection pre.@'sures high enough to cause reservoir fracturing so that in.iectivity can be obtained. The reservoir adjacent to the well becomes heated and then, subsequently, the flow is reversed to @illow I)roduction. This continues until the flow has declined to uneconornic values and then the cvcle is
repeated. This is the process @hich has been used by Imperial Oil in its large and suc- cessfut ccimmercial project at Cold Lake@6.7.8). The process is rela- tiveIN, efficient and oil flows of the order of 100 barrels per day per well @ire obtained with oil/steam ratios of about 0.3. A major adN aiitagt@ of cyclic steaming is that the heat tends to be concen- trate(i where it can do the most good, i.e. nearer to the wellbore. The main problems with the method are associated with the high well den@;ity required (about 1.5 ha per well) and the relatively lolk, economic recovery ( 1 5-20%).
.,inlierent limitation of the cyclic steaming method is that there is little drive available to move the hot tluids to the well. It is th(iught that some of the drive comes from the recompaction of the fractured reservoir and some from gravity drainage around the well@'ll. The probleni with gravity drainage, in this application, is th,,it the conventional (i.e., near vertical) wells which are employe(i have a limited contact with the reservoir and the need foi i-@idial flow provides a considerable resistance. Flie steam-assisted gravity draina-,e approach which is re@ iewedl in this
paper employs gravity drainage to move the crL[dC to the production well but the contact with the reservoir is MLICII grc-ater because horizontal wells, and potentially very long horizontal wells, are emptoyed-, these have much greater contact with the reservoir than do conventional wells and adequate flows can he achieved with heads equivalent to that obtained from gravity: this is not possible with vertical wells. lt is the use of horizontal A,ells which allows oil to be produced al economic rates with the main driving force being gravity. The process allows the oil to 1'ellI@tin h-,)t
as it drains downwards and there is a systematic cover- agc of tht- reservoir so that high recoveries can be achieved. The Initial Concept The mechanism by which the process proceeds and the general natui-e ol' the flows within the reservoir are indicated by the dia- gr@iiii in Figure 1. This shows the process in a fairly early phase. StL'illn is being injected from a well, either a horizontal well, of- The Journal of Canadian Petroleum Technology

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"STEAM-ASSISTED GRAVITY DRAINAGE: CONCEPT,

DEVELOPMENT, PERFORMANCE AND FUTURE"

this article begins on the next page F

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