Accurate Two-phase Oil Well Testing At High Water-oil Ratios
Authors: Chekerda L.B. , Johancsik C.A. 
Published: March 2010

Industry standard well testing equipment using two-phase separation is often inadequate when used to test wells which produce at very high water-oil ratios. In the Redwater field, which currently produces at an average water:oil ratio of 115:1, two-phase tested wells have had their oil production overstated by as much as a factor of two. This paperdescribes inexpensive equipment and procedural improvements that were made to two-phase well testing systems in the Redwater field that resulted in dramatic improvements in well test accuracy.

Background

The Redwater oilfield was discovered in 1948 by Imperial Oil Limited. Early development was rapid and by 1952, 975 wells had been drilled on 40 acre spacing. Oil production comes from a Leduc age carbonate reef located some 945 m beneath the surface. The pool, which is supported by a strong bottom water drive, contained 1.3 billion barrels of original oil in place of which 63.5% is recoverable under primary production. Large submersible pumps were installed in' the 1970s to increase production and since the early 1980s the field has experienced a harmonic oil production decline and a continuing rise in water production. Oil production today is 1,500 m3/day compared with the peak rate of about 20,000 m3/day in the early 1970s. Water production is currently 170,000 m3/day for a field average water-oil ratio of 115: 1.

The pool is not unitized but rather is competitively operated by 12 companies. The four major companies, who together own 96% of the pool, share a common gathering system and fieldgate battery where custody transfer occurs. Crude sharing is based on individual well tests conducted by each operator.

The facilities installed to conduct well tests vary widely among producers. While some tests are conducted using three-phase test separators which separate oil, gas, and water into three distinct and separately metered streams, many are conducted using two-phase (liquid and gas) separators and sampling equipment. It is the two-phase facilities in particular that have proven to be inadequate in providing accurate well tests, particularly at the high water-oil ratios experienced in the Redwater field.

Two-phase Well Test Equipment

Two phase oil well testing equipment involves a two-phase separator which separates liquid and gaseous phases, and an automaticsampling system which is used to determine the water and oil content of the liquid flow stream. There are many (perhaps 20 or more) designs of samplers that are commonly used in well testing applications. In Redwater, these standard oil field equipment setups have proven to be inadequate for the intended application.

The American Petroleum Institute (API) does not provide any recommendations on well testing or on sampling as it applies to well testing. In the API Manual of Petroleum Measurement Standards: Chapter 8-Sampling(1), there are standards for sampling of petroleum and petroleum products. However, these guidelines apply to custody transfer measurements of homogeneous pipeline products where the maximum water content is not greater than three to perhaps 5%. In well testing applications it is very typical to have to sample fluids with significantly higher water contents.