Openhole completions with prepacked screens are increasingly the completion of choice for horizontal wells requiring sand control. For maximum productivity from these wells, preventing mud damage to the fonnation and the screens or removing it before bringing the well onto production is vital. A common industry approach to this problem is to displace the drilling mud to an aqueous clearfluid before production. typically followed by a breaker fluid. This paper details an alternative approach: bringing the well on without cleanup. The paper outlines the decision-making process, shows how the drill-in-fluid and cleanup operations are designed, explains how laboratory testing can be used to support the design philosophy, and highlights that quality control of the mud system while drilling is critical to the success of such a well. Case histories in two very different reservoirs are presented that illustrate the success of this approach.
This paper describes a major joint industry study into the effectiveness of mud clean-up techniques for horizontal wells involving eight oil and service companies. Experimental work was carried out on a small scale laboratory clean-up rig to quantify mud damage and test breaker effectiveness. Large scale flow loop testing was used to evaluate overall clean-up techniques including displacement efficiency and damage to pre-packed screen completions. Tests on a wide variety of mud systems showed surprisingly little difference in the performance, from a formation damage perspective, on clean sandstone cores. Oil based muds were, however, found to produce less damage than water based muds. A range of breakers were tested with each mud system. There was a large variation in the results of breaker testing. with breakers, unexpectedly. sometimes causing an increase in the pressure required to break through mud filter cake and/or formation damage. Whole mud was found to cause significant damage to pre-packed screens; this damage was sometimes, though not always. removed during clean-up. Poorly centralised screens were found to result in large amounts of mud and debris on the low side of the hole. Introduction Horizontal wells are increasingly used in field developments to maximise well productivity, access reserves, or reduce water and gas coning by reducing drawdown. These benefits of horizontal wells can only be attained if all well sections are flowing without significant near wellbore damage. Most horizontal wells are completed open hole i.e. without a cemented and perforated liner. These open hole horizontal wells, which are typically completed with prepacked screens or slotted liners, differ from conventional cased and cemented liners in two important ways:–Oil or gas must be produced through mud filter cake and mud induced formation damage as perforations are not shot through the damaged layer.–Sand face completions such as pre-packed screens designed for sand control are themselves susceptible to damage from the mud system. A variety of specialised mud systems and clean-up techniques are used to try to minimise mud damage or remove it during well completion operations. The most common approach is to use a brine based mud system with an acid or water soluble weighting agent and then use acid breakers to dissolve filter cake solids and polymers. Brine is used to reduce mud solids loading and new systems are continuously being developed to extend this low-solids approach to higher mud weights; however, little work has been carried out to establish the need for, or the effectiveness of clean-up procedures. Radically different approaches to clean-up have been applied to the same mud system with apparent success. Also, wells in which little or no clean-up was attempted, have been completed with minimal damage. To address these issues an eight partner joint industry project was set-up to investigate mud clean-up in horizontal wells. A programme of experimental work has been conducted that studied the clean-up of a number of different mud systems, using both laboratory and large scale equipment. Additionally, large scale experiments were carried out to study the displacement process in horizontal wells. The objective of the project was to try to understand the parameters which control mud damage and its clean-up with breaker systems and thus allow a more systematic approach to fluids selection and the design of horizontal well completion operations. This paper describes the main findings of this joint industry project. Work Programme The work programme for the joint industry project consisted of seven test series. Each of the first six test series investigated the damage and clean-up behaviour of a specific mud system with a number of breaker systems. Both small scale laboratory tests and large scale flow loop tests were used. The final test series was a displacement study. P. 801
A new computer based cementing simulator is described that quantifies the effect of key displacement variables such a s geometry, standoff, fluid rheology and density, and flow rate. The paper shows how to optimise cement placement within the fracture and pore pressure constraints of the well and demonstrates its use through case studies from the North Sea. The technique assesses the quality of mud removal and cement placement as well a s estimating the circulating pressures throughout the job. The examples illustrate how existing rules of thumb can be confusing and how reliable cement jobs can be achieved through quantitative design procedures.
Over the last 20 years of Dolphin field gas production, it has always been a challenge to estimate inplace volumes, history match pressure and water productions, and generate a reasonable range of production forecasts. Given the unconsolidated nature of the Greater Dolphin Area (GDA) reservoirs, it has not been possible to acquire representative cores and perform successful core plug tests. As a result, estimates of not only static parameters like porosity and saturation, but also dynamic parameters such as compressibility and permeability have been challenging. Historically this led to smaller static volume estimates than dynamic volume estimates from material balance. A new set of static and dynamic GDA reservoir models have been built that integrates and incorporates reservoir engineering techniques and revised petrophysical/geological properties to resolve the mismatch between static and dynamic volumes. Key inputs of this new model are in-depth reviews of dynamic datasets ranging from compressibility, Production Logging Analysis (PLA) results, and both core and well test derived permeability. Revised interpretation of these datasets, along with an iterative approach between static and dynamic QC has resulted in a range of deterministic and probabilistic history matched reservoir models that are used for forecasting and project planning decisions. The main approach was initially focused on reviewing the material balance studies and evaluating the effect of compaction in these unconsolidated sands. This highlighted the impact of compressibility in partially inflating the historical estimates of dynamic volumes. Additionally the study was focused on de-convolving field-wide material balance of comingled production from multiple reservoirs with heterolithic formations and a varied range of static properties and pressure depletion trends. Later the level of communications amongst various compartments and with nearby fields were investigated and taken into account. This assisted with an improved understanding of volume distributions amongst different reservoirs/compartments and helped to constrain the connected volumes, while building the dynamic models. In addition, a new methodology was developed to model permeability based on the kh derived from pressure transient analysis and the layer contributions observed in initial PLA results. This novel permeability modelling technique also helped to match PLA results in wells and individual reservoir layers gas contributions and water productions. These new approaches along with an alternative petrophysical methodology to better estimate water saturation within thin bedded intervals have been incorporated into an integrated workflow to account for both static and dynamic uncertainties. This set of probabilistic simulation models achieved a range of history matched results with a better understanding of dynamic reservoir behavior and also helped to overcome the historical shortage of static volumes required to match observed pressure data. This in itself brought more confidence towards generated production forecasts and future project's decision making processes.
The Dolphin Field has been producing gas since 1996, however predicting in place volumes, reserves and forecasting production has been a challenge since field inception. The fact that in place estimates have increased significantly since development sanction highlights that a range of geophysical, geological and petrophysical uncertainties are associated with the field. Historically, static volumes have been smaller than dynamic volumes estimated from material balance. The explanation of this difference traditionally related to uncertainty in contact depth (given the minimal data on contacts), that adversely caused poor predictions of water production in the historical models. Many of the reservoir units within the Greater Dolphin Area (GDA) are characterised by a heterolithic deltaic succession of centimeter scale very-fine sandstone, siltstone and mudstone. Given the thin-bedded nature of the reservoir, conventional wireline-logging tools lack the resolution to accurately resolve many of the static parameters including water saturation. However, based on the available PLT data, it is believed that these thin-bedded intervals generally contribute to the production from the wells and hence to the fluid flow in the reservoir. A new static and dynamic reservoir model of the GDA has been built that integrates and incorporates new seismic interpretation, petrophysical recharacterization, revised geological and reservoir engineering concepts, and eventually history matching to production data. A key component of this new model build has been integrated modelling iterations amongst different disciplines from new petrophysical interpretations through to dynamic simulation. Initial iterations used a conventional formation evaluation method and resulted in simulations that showed accelerated pressure drops (compared to production data) as a result of failure to capture flow from thin-beded intervals. An alternative petrophysical methodology that aims to better estimate water saturation within thin bedded intervals has been incorporated into a new workflow to account for the thin bed volumes. The new thin bed simulation model results in greater gas contributions from the thin-bedded intervals and helps overcome the historical shortage of static volumes required to achieve a pressure match.
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