PRESENT-DAY SOLUTIONS FOR FAILURE-FREE DRILLING OF HORIZONTAL WELLS IN COMPLICATED GEOLOGICAL CONDITIONS

PRESENT-DAY SOLUTIONS FOR FAILURE-FREE DRILLING OF HORIZONTAL WELLS IN COMPLICATED GEOLOGICAL CONDITIONS

Tailakovskoye field is located in the southern part of Surgut sub-district, Khanty-Mansiysk Autonomous District (KhMAD), Tyumen region. The section productivity is relating to the Achimovsky deposits of Lower Cretaceous (beds Ach3_4) and Jurassic (beds J1, J2, J3, J4). Beds of group J2-3 being developed now contain about 90% of initial recoverable reserves.


The field development started in 2004. Major productive sediments of Tailakovskoye field were drilled mainly by a pattern of directional wells (DW). At the current field development stage, development drilling of beds J2-3 by a pattern of horizontal wells (HW) is employed for the strategic purposes of maintaining the production, which enables higher and more profitable production rates. OAO Slavneft-Megionneftegaz has implemented modern processes of multistage hydraulic fracturing (HF) as additional production stimulation methods.


RANGE OF PROBLEMS


Drilling of horizontal wells in a group of beds J2-3 is connected with risks of operating problems associated with adverse geological factors, namely, instability of the section in Bazhenov formation and roof of Georgiev formation.


This interval is unstable due to regional and general tectonic settings in the work area: existing local fault tectonics (Upper-Vasyuganskiy deep fault) in north-west and south-east parts of the field, derived during formation of Kaimysovskiy and Demiyanskiy arches tied to Koltogorsko-Tolkinskoy suture zone.


Based on interpretation results of 3D seismic survey carried out in Tailakovskoye field in 2005-2006, some authors interpreted the identified disruption zones as tectonic faults.   


General tectonic settings are suggestive of existing range zones of lateral branch disjunctive structures and fracturing zones complicating geological structure of beds JC1-JC4 and spreading from platform mantle gross intervals to the roof of Jurassic sediments.


In terms of horizontal well drilling technology, the field tectonic structure predetermines risks of accidents and problems connected with the open hole instability in the interval from the roof of Jurassic sediments to Bazhenov and Georgiev formations. This interval disruption during drilling is caused by developed microcracks, abnormal distribution of geomechanical stress in the rock matrix. Physical properties of late lithogenesis argillites in Upper Jurassic sediments and mechanics of their disruption during penetration significantly limit potential efficient solutions for inhibiting and stabilizing the open hole.


The most significant abnormal geomechanical loads arise during penetration of this interval by a tangent section at high inclination angles (75-87°). Drilling complications result in the open hole restriction, hole wall sloughing, BHA sticking, and production time losses for the open hole additional gauging and reaming. The most complicated cases can result in inability to run the liner to a target depth in the unstable argillite interval, which complicate drilling of a subsequent horizontal section in the uncased borehole as well as in re-drilling of the tangent section.


A share of horizontal wells (HW) in the field increased in the period of 2011-2015, thus resulting in gradual reduction of operating problems connected with the open hole instability in the interval of Bazhenov and Georgiev formations. Reduction in failures is explained by accumulated drilling experience, improved well drilling procedures, but, to a greater extent, the open hole stability was ensured by engineering solutions in the area of drill muds with a general development history represented as a search for optimal system types and determination of the most efficient mechanism to prevent occurrences of wellbore instability.


Period

Applied technologies

Key parameters

2006-2007

Base polymer-carbonate potassium chloride mud

Specific gravity while penetration to Bazhenov formation – 1.10-1.14 g/cm3

Mineralization – 15-20 g/l СL-

2008-2012

Base polymer-carbonate  potassium chloride mud

Gradual increase of specific gravity while penetration to Bazhenov formation to 1.18-1.20 g/cm3

Mineralization increase to 20-40 g/l CL-

2013-2015

Freshwater systems based on synthetic polymers

Specific gravity increase within a range of 1.20-1.32 g/cm3.

Application of inhibitors-microcolmatants











Another impetus to develop drill mud engineering was provided by a new technology development partner represented by AKROS oil service company engaged by Slavneft-Megionneftegaz.


An algorithm to solve the problems of the open hole instability in the Upper Jurassic interval was selected based on laboratory studies of actual core material and structural empirical analysis of over 200 wells drilled.


The laboratory studies of core material and the work to select an optimal inhibiting technique identified the following key principles and characteristics:

argillaceous rock activity described by cation-exchange capacity is minimal;

- a degree of dependence of sample hydration on drill mud mineralization and presence of ionic inhibitor is low.

- linear increase of rock volume (Linear Swell Meter Test) insignificantly depends on the type and concentration of ionic inhibitors (salts, amine derivatives)


The Upper Jurassic sediments penetrated by drilled wells were analyzed to identify a dependence of the interval geomechanical stability on directional attitude of the tangent section distance.

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In 2015-2016, pilot testing of various inhibiting systems were carried out to test and identify the most efficient methods of ensuring the wellbore stability (Megionneftegaz-vesti newspaper, issue 20 of July 3, 2015).

 

As a result of conducted studies, specialists of the analytical center at AKROS oil service company issued recommendations for ensuring the open hole stability and identified the optimal inhibiting mechanism aimed at micro-colmatage of fractured rock by combined inhibitor MEX-WSP (wellbore stability product).

In addition, hydrodynamic simulation in the MUD OFFICE software resulted in identification of a range of permissible hydrodynamic pressures and optimization of the flush mode.

 

At present, the integrated approach has been implemented to ensure the open hole stability when drilling a number of wells at Tailakovskoye field pads 8bis and 42bis.

The implementation resulted in zero problems connected with the open hole instability in the interval of Bazhenov and Georgiev formations.

A total construction period of horizontal wells have been reduced by 6-8% due to eliminated operations of the tangent section reaming and increased rate of round trip operations with no draggings and settings.


In addition to reduced construction periods, implementation of proposed solutions significantly mitigates risks of BHA sticking, accidents, and loss of an open hole section. 

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AUTHORS:

Ilyichev S. А., managing director of oilfield services Slavneft'-Megionneftegaz OJSC

Nazarenko V. V., chief specialist Slavneft'-Megionneftegaz OJSC

Popov S. V.., technical manager AKROS LLC

Source:  "OIL & GAS JOURNAL RUSSIA", 2016, №10