Directional drilling in coal has been used in Australia since 1995, however its use only became widespread from 2002. Since then its growth has been rapid, and lessons are often being learnt by each and every new operator or contractor attempting the art. Directional drilling refers to intentionally deviating a wellbore from the path it would naturally take. Here, all types of well directional drilling, from deviated vertical wells to horizontal sections will be considered.

An Australian history of directional drilling

Directional drilling rigs were introduced into Australia in the late 1980s by AJ Lucas’ Andy Lukas for a job at Appin Colliery in a joint venture between Occidental and AGL. The rig was introduced to test the effectiveness of the then new technology.

AJ Lucas decided to continue with directional drilling. The first horizontal directional drilling (HDD) job performed by AJ Lucas was duplicating an oil pipeline under the taxiway at Sydney Airport for Caltex. Market interest in HDD began to grow and continued into river crossings as Telstra, known at the time as Telecom, rapidly expanded its fibre optic network. Competitors joined the market in the early 1990s. Pontil, later Major, embarked on lateral coal seam gas (CSG) drilling at Moura in 1995 for BHP Seamgas. Initially the work was performed to pre-drain methane ahead of mining, but was later changed to CSG production and led to the first gas piped to market in Gladstone.

Until 2002, directional drilling within Australia continued to grow via the HDD market, with the CSG market seen as a mere curiosity by many. Since then, the CSG market has grown rapidly. Directional drilling can provide many advantages when drilling for CSG, such as:

• Reduced environmental impact (more drainage area per well);
• Less surface infrastructure due to more concentrated operations;
• Simplified land access;
• Better exposure to overall gas reservoir;
• Geological reasons (faults, structures, etc);
• Access to optimal coal cleating orientation;
• Ability to tap into lower permeability areas; and,
• Rigs can be smaller and easier to move, not traditional larger oil field-type spread.

Before CSG directional drilling can start

There are three rules to be remembered when considering directional drilling in coal:

• Coal/CSG exploration, drilling, production and workover is a system, and must be treated as such;
• The overall gas production result is a function of how successfully all parts are executed; and,
• Any damage as a result of any part of the work will affect the final result.

To make directional drilling work, fundamental inputs need to be obtained, such as a geological model, a permeability model, a gas-in-place model, and a standing water level model. From this, outputs need to be created. This involves a complete geological model including a coal seam thickness contour map and any known and suspected structures or faults; a gas-in-place contour map; and, a hydrogeology/water reservoir map, including potential recharge direction.

From here, the CSG wells to be drilled are designed. The well planning is an iterative approach where one change affects other parts of the drilling program. A principal design should be created, and as many iterations as necessary should be performed to finalise all answers and understand all risks and limitations.

Well planning is dependent on strong fundamentals for good results, and each field must be designed individually because of the variability of coal reservoirs. In addition, vertical wells, both normal and deviated, are generally used until the primary geological model is proven. However, 3D seismic may improve confidence to use lateral techniques earlier.

Limitations of directional drilling

CSG drilling is mostly a logistics exercise because the wells are drilled so fast. As such, the rig spread and personnel must be efficient in all aspects to achieve commercial viability of a project. Bigger is not always better, and rigs should be sized appropriately to avoid high costs.

Larger rigs are required if drilling wells with total depths around 1,700 m and 2,800 m. This is due to drill pipe limitations, principally the onset of pipe buckling.

Directional drilling in coal is not an exact science yet, with interpretation ‘on the fly’ still required for best results.

Primary limitations are:

• Gamma sensors used for geosteering require a strong transition to ‘see’ more than 500 mm from the sensor toward the floor or roof interface;
• High ash coal will reduce gamma sensitivity, sometimes making it unusable for geosteering;
• Survey stations carry accumulated error. This error cone gets bigger the further you progress downhole until you ‘range’ at or near a vertical well. This affects position confidence; and,
• Geomodels are a series of assumptions between known points. Directional drilling tests every one of those assumptions.

Managing downhole risk

It is strongly suggested that a trigger action response plan (TARP) or a decision tree be jointly developed before a job starts, with input from the drilling and service companies to:

• Decide how long you will spend exploring, fishing for stuck/bogged equipment, branching, and attempting to intersect a vertical well; and,
• Decide how you will treat tooling failures.

For the best directional drilling results, understand and exploit rig capacity versus cost for the project, and use people experienced in CSG drilling in the areas of geological and reservoir evaluation and drilling and completion.

In addition, be prepared to experiment by trying different drilling and completion techniques, and continually update plans based on real data rather than assumptions.

CSG next generation development

Drilling plays a crucial part in the success of CSG production, and how the well is drilled has a far greater affect on final production than traditional oil and gas reservoirs.

Currently, CSG development is concentrated via vertical wells in high gas areas. However, an increasing requirement exists for directional drilling expertise, as the easily developed high gas areas will be drilled and producing in the next five years. There are plenty of commercially viable lower gas areas or deeper reservoirs existing in Australia and these benefit the most from directional drilling to enable cost effective production.

Directional drilling in coal has reliably reached total depths of 2,500 m, and managed pressure drilling of coal, pioneered by Lucas, is rapidly developing as an accepted improved method of directional drilling. Lucas has recently completed engineering work for a 4,000 m directionally drilled well and is negotiating for a trial site.

Batch production-style directional drilling will be developed and will rise in importance as the full impact of LNG plant gas requirements become clear, thus dictating the large numbers of wells needed to supply CSG feed stock.

Environmental concerns surrounding water production and treatment are becoming more important and further favour directional drilling so that treatment plants can be concentrated in one area.

Over the coming years partial equipment automation will improve repeatability and cater for reduced skilled personnel.