Although not routinely cored, horizontal wells are usually drilled ill developed fields where good core coverage exists. This provides an opportunity to reevaluate pre-existing cores for detailed analysis. Additionally, formations can be pilot-hole drilled and cored, prior to going horizontal. For non-coredreservoir sections, drill cuttings can be valuable in formations evaluation.
Areas that should be assessed before horizontal drilling include, reservoir description, wellbore stability, and formation damage potential.
Reservoir description, including heterogeneity definition, is more difficult in horizontal wells when they are not cored or logged. Current evaluation methods include thin-section petrology and mercuryinjection on drill cuttings from the horizontal well.
Where difficulty is encountered in identifying fluid contacts, thermal extraction chromatography (TEC) can be performed. Petrology, capillary pressure, and TEC analyses, require accurate sample and cuttings collection at the well surface. Automated cuttings samplers provide a superior collection method overmanual methods. Though horizontal wells are not usually logged, gamma ray devices are frequently run in conjunction with MWD tools. Transforms can be developed between the gamma response and permeability from existing offset cores. High resolutionpermeability logging devices can be utilized, which provide continu.ous, log-profile data for more accurate permeability models.
Wellbore stability analysis is performed by either dynamic or static methods. These tests provide mechanical properties for describing the rock "failureenvelope", and predicting safe operating conditions during drilling and production.
Formation damage testing is typically performed on existing cores from offsets, and involves petrology and ynamic /leakoff/regain permeability testing. A key use of damage is mud solids invasion, which can be assessed and optimized using mercury injection oncore or cuttings, together with panicle size analysis of drilling mud solids.
The applications for horizontal well technology are numerous including:
intersecting (or avoiding) natural fractures,
mitigating coning,
accessing low permeability reservoirs,
reducing production related problems such as scale, sanding and fines migration,
improved EOR (thermal and miscible),
accessing offshore and environmentally sensitive locations, and
gas storage.
With all these applications, there are common objectives:
accelerated production,
increased recovery, and
improved cashflow.1 Given the rapid growth of horizontal drilling, itappears that industry is achieving these objectives, despite recent statistics which indicate that 35 percent of horizontal wells are still uneconomic. .1 Considering the industry success rate for all wells (development and exploratory) was 77.6 percent2 in1994, 35 percent is not that favourable, especially when one considers that horizontal wells are normally drilled in developed areas.
Several authors.3–8 have recognized the need for a thorough reservoir description through a multidisciplined approach, in order to achieve high success rates with horizontal wells. The additional complexities and uncertainties introduced by horizontal wellbores make convectional analytical techniques, such as electrical logging.9–12 and pressure transient analysis.13–15difficult. Assumptions regarding flow geometry and productivity predictions become uncertain.16 Joshi.17 emphasizes that an u
