Relating To Injection Wells

Abstract

A method for determining cross-flow in an injection well in a multi-layered reservoir. Wellhead pressure measurements are made and the characteristic curve analysed to identify late wellbore storage indicating cross-flow. The water hammer pressure wave can also be analysed at shut-in to identify the presence of sand fill in the rat-hole. By measuring the wellhead pressure, intervention-less and continuous monitoring of the injection well is available so action can be taken to reduce sand production and maintain injectivity.

1 . A method for determining cross-flow in an injection well, comprising the steps:
 (a) injecting a fluid into a well;   (b) shutting-in the well;   (c) measuring pressure at the well;   (d) constructing a characteristic curve; and   (e) identifying late wellbore storage.   
     
     
         2 . A method according to  claim 1  wherein the pressure is measured at the wellhead. 
     
     
         3 . A method according to  claim 2  wherein pressure is measured continuously during shut-in. 
     
     
         4 . A method according to  claim 2  wherein the pressure is measured at a frequency of around 1 Hz. 
     
     
         5 . A method according to  claim 1  wherein duration of cross-flow is measured on the characteristic curve by the time duration of a slope of the late wellbore storage. 
     
     
         6 . A method according to  claim 1  wherein step (b) is a hard shut-in. 
     
     
         7 . A method according to  claim 1  wherein a water hammer pressure wave is created at shut-in. 
     
     
         8 . A method according to  claim 7  wherein a last injection rate of fluid into the well at step (a) is calculated to provide sufficient water hammer amplitude without damaging the well. 
     
     
         9 . A method according to  claim 7  wherein attenuation of the water hammer pressure wave is analysed to identify the presence of fill in the rat-hole. 
     
     
         10 . A method according to  claim 9  wherein rapid attenuation of the water hammer pressure wave shows that the rat-hole is filled with debris. 
     
     
         11 . A method according to  claim 9  wherein reflection of the water hammer pressure wave shows that the rat-hole is free of debris. 
     
     
         12 . A method according to  claim 9  wherein measurements of the wellhead pressure are used to both detect sand-fill in the rat-hole and determine cross-flow in the well. 
     
     
         13 . A method according to wherein the method includes the initial step of establishing an injectivity index decrease across shut-in. 
     
     
         14 . A method according to  claim 1  wherein the method includes the further step of performing a sand strength analysis to establish risk of sand production. 
     
     
         15 . A method according to  claim 1  wherein the method includes the further steps of isolating one or more injection intervals and/or enforcing procedures of progressive shut-in.