The Selective Evaluation and Stimulation of Horizontal Wells Using Concentric Coiled Tubing
Authors: Fried S.J. , Bell P.G. , Sask D.E. , Ranks M.R. 
Published: December 2007

Abstract
The flow testing and evaluation/stimulation of horizontal wells in oil and gas reservoirs is an important process used by operators in both openhole and cased hole applications. The information gained from openhole flow tests; permeability, flow rates, skin damage and water production is used to calculate well deliverability and helps confirm the appropriate remedial activity.
Although well testing is common in all reservoirs, the testing of sweet or sour horizontal wells is still a significant challenge for operators and service companies alike. The overall length of the horizontal section 300–4500 ft (100–1500 m) results in production of multiple intervals making flow and pressure transient analysis difficult. In addition, the testing of sour wells has been very limited due to the concerns of H2S embrittlement of conventional drill pipe or tubing. In horizontal wells the challenge is to safely, quickly and selectively test the horizontal section of the well and use this information to implement remedial stimulation or workover to improve hydrocarbon production.
This paper will review a new technology which uses an inflatable straddle packer "Drill Stem Test" (DST) tool deployed into the horizontal well using a "Coil in Coil" coiled tubing string configuration. An electrical conductor is located inside the inner string which allows for "real time" formation evaluation and tool operation. The inner coil string is used for all well flow and stimulation operations with the coil in coil annulus utilized for circulation operations and packer element inflation. More importantly, the outside string also provides for pressure monitoring, flow containment and well control in the unlikely event of a failure of the inner string. Using the test tool, the operator can set the straddle packer on a variable length 12–150 ft (4-50 m) of openhole zone and conduct a flow test followed by a pressure build-up for pressure transient analysis. If the evaluation of the real-time data indicates a wellbore skin, the same tool can be used to selectively stimulate the formation; while monitoring bottom hole pressure, without unsetting the packer and then repeating the flow or pressure buildup test. Since the evaluation/stimulation process from reservoir to separator is "closed loop", the production optimization of long horizontal sections can be implemented in a more cost effective and safer manner.

Introduction
Horizontal drilling has emerged as an important technology in the Canadian and international oil and gas industry. As operators pursue the continued development of more challenging reservoirs, using both new and existing wells. Horizontal wells are typically used to: intersect more reservoir to increase well deliverability, reduce the pressure differential to the wellbore allowing additional production from pressure depleted reservoirs, or avoid the production of bottom drive water zones.
As horizontal drilling applications have increased so has the debate on the differences between conventional overbalanced and underbalanced drilling (UBD) technologies. The drilling of the horizontal well results in the openhole zone being exposed to drilling fluids and cuttings for extended periods of time versus vertical well development. In the vertical wellbore the extent of damage is limited and can usually be removed using cased hole selective stimulation and evaluation techniques. P. 493