Abstract

Coiled Tubing (CT) cementing has been widely used and highly successful for remedial squeeze and plug back operations for over 20 year's[1,2,3].However, the vast majority of these wells were at deviations less than 90 degrees.

A long horizontal well in the Alpine field on the North Slope of Alaska was drilled early in the development phase and was out of pattern (Fig. 1).The well required a plug back and sidetracking to maintain desired off-take strategy (Fig. 2).The well was drilled to a total depth of 11,984 feet and completed with approximately 2,050 feet of 4–1/2" slotted liner inside the 2,210' of 6–1/8" hole.Near the middle of the horizontal section, the well's deviation climbed to a maximum of 96 degrees.

Cementing operations have long been recognized as a problem in horizontal wells.However, a search of the SPE online library identified only 5 papers that mentioned the challenge we faced while a search of "horizontal" yielded 5,534 hits.Although the 5 papers did discuss the problem and gave some general guidance to cement design consideration, there was little specific information on the "best practice" approach to Plug and Abandon (P&A) long horizontal wellbores.

Based on the successful CT squeeze program in Alaska, a team of engineers and field supervisors decided to use CT cement squeeze technology to seal the lateral portion of the wellbore and leave a cement base for subsequent sidetracking operations.This paper will discuss the details of this job including:

  1. Job Planning

  2. Cement design and testing

  3. Tools and Equipment

  4. Wellbore geometry and Placement details

  5. Onsite Job Execution details

  6. Results

  7. Lessons learned

Field Overview

The operator, ConocoPhillips Alaska Inc., and partners in 1994 discovered the Alpine field.The Alpine field is located in the Colville River Delta a few miles south of the Arctic Ocean and approximately 70 miles west of the Trans Alaska Pipeline.The facilities are connected to the North Slope road system via ice road for approximately three months of the year.Aircraft provide the only mode of transportation at other times.Produced crude is transported to the Trans Alaska Pipeline via the East-West running Alpine and Kuparuk common carrier pipelines.

The reservoir was under-saturated at discovery with a gas oil ratio of about 850 SCF/Bbl.The produced oil gravity currently averages 39º API.As a result of the depositional environment and minor fault offset, excellent vertical permeability is observed and the productive sands are pressure connected across large distances.A "water alternating miscible gas" flood is being conducted in the Alpine reservoir.The field is developed with line drive patterns, utilizing horizontal producers and injectors in a one to one ratio (Fig. 2).

Given the high mechanical strength of the clean fine-grained Alpine sandstones, the operator elected to leave the horizontal sections on the wellbore uncased, to minimize the chance of formation damage.A 7.0" intermediate casing shoe is set just below the top of the producing formations at high angle.The uncased horizontal production hole typically extends 3000–4000 feet beyond the intermediate casing shoe.Production and injection tubing is primarily 4.5" although some lower rate wells are completed with 3.5" tubing.A production or injection packer is located a few hundred measured feet above the casing shoe.A typical Alpine well completion is shown in Figure 3.

While performing above expectations, this completion practice has proven to be a difficult environment to access.Early attempts at logging these wells using CT memory tools and conductor line tractors provided less than ideal results in reaching the TD in the extended horizontal open-hole sections.Drilling and formation debris and abrasive formations in combination with wellbore geometry limited our ability to reach the full measured depth of the lateral sections.Portions of open-hole lateral sections with fill or debris is not the only potential challenge to successfully cementing these wells; recent caliper logs suggest that the wells may also have sections that are washed-out or significantly out of gauge.

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