Successful underbalanced operations demand a systematic approach with integration of engineering, operations, production, reservoir, and geology. There are significant advantages to underbalanced drilling (UD). Formation damage can be minimized if proper circulating fluids and operating parameters are chosen; there is an opportunity for reservoir evaluation and characterization while drilling; lost circulation and differential sticking can be avoided, combined with improvements in drilling rate of penetration (ROP); and it also provides an alternative method of well control.This paper discusses engineering challenges and considerations for underbalanced operations using multiphase flow as well as advances made during UD with rotary jointed pipe. Excluding air, mist, and foam, underbalanced techniques with multiphase fluids are reviewed. Much of the discussion and concepts also apply to coiled-tubing underbalanced operations.

Copyright 1~, lA~SPE Drilling tifa This paper was prepared for prewnt8tion q t the 1~lAOC/SPE Drilling Wferenm held in Dallas, Texas M March 1S98, This papsr wc -Iected for pre=ntation by an lA~SPE Progr8m -mittes follting review of information contitned in an abtict submittsd by h author(s), Contents of the paper, as presented, have not~n rsviawsd by the International Association d Drilling Contractor or the We& d P*leum Engineer8 and are subject to corr-"on by the author(s), The material, as presentsd, dms not nsces-rily reflect any position of the IAOC or SPEf their officers, or members. Papar$ presented at the IAWSPE meetings are subject to publication revisw by Editorial Mmitteas of the IADC and SPE, Electronic reproduction, distribution, or storage of any part& this~~r for commercial purposes without the wi~en consent of the Scciety of Petroleum Engineers is prohibited, Permission to reprcduce in print is restricted to an abstract of not more than~words, illustrations may not be copied. The abstraot must contain conspicuous ackdedgment of Mere and by whom the papsr was presented. Write Librarian, SPE, PO, h @32S3e, Richardson, TX 75C333SW, US.A, tix 01-972-952.9435, AbstractA part of the planning of underbalanced operations is to estimate drilling and production related parameters. Wellbore pressure and fluid phase velocity profiles, hydrocarbon production management mechanisms and injection gas requirements are necessary for designing underbalanced drilled wells. A steady state underbalanced drilling sofiware tool has been developed by Shell to assist their well engineers in planning and executing underbalanced operations. The simulator has been validated and its accuracy proven with field data comparisons. The accuracy was within the range of the intended area of application and has indicated less than 10% difference in measured wellbore pressures. The objective of the software development is to release the simulator to the industry. Concurrently the model will be developed further and its accuracy improved. A concern at the moment, is the limited number of wells drilled underbalanced with stilcient data gathered to allow extensive simulator comparison, To obtain an overview of capabilities and applicability of presently on the market available simulators, a thorough comparison between models is recommended.

Is this the beginning of the end of weighted mud systems? An advanced well control practice called Managed Pressure Drilling (MPD) is staged to challenge the conventional drilling practice of when in doubt "weight it up". Formation overpressures have traditionally forced operators to weight-up mud systems in order to advance drilling operations while preventing formation fluids invasion. A direct consequence of increased mud weight is a dramatic reduction in drilling Rate of Penetration (ROP), requirement for additional casing seats/strings and increased well control risks due to kicks from losses. This drilling hazard has represented a major source of invisible or intangible lost time and hence, cost. A drilling hazard mitigation multi-well trial was carried out to investigate and quantify the reduction in ROP as a result of mud weight increases to overcome troublesome formation overpressures and associated High Pressure Low Volume (HPLV) nuisance gas. Without increasing mud weight to control overpressures, MPD technology was applied to safely and cost effectively drill through overpressures and avoid an intermediate casing string normally used to isolate a loss zone. MPD trial results examined in this paper addresses many of the issues and provides forward-looking statements regarding large-scale introduction of the system to other fields. The multi-well trial results showed a close match to the predicted MPD ROP curve, achieving an economically rewarding ROP gain of at least 2.5 times and gross well cost reductions of over 20%, without recordable troublesome zone Non-Productive Time (NPT). Initial concerns of borehole instabilities using lightweight mud with borehole pressures less than the adjacent pore pressure were not observed (in both vertical and directional well cases). Introduction MPD technology is challenging the traditional drilling practice of weighting a mud system while drilling formation overpressures. MPD practices are suggesting that weighting up of a mud system is a major source of invisible NPT. Non-Productive Time (NPT) It has been reported by Dodson(1) that 25% of an average well's cost is recordable drilling NPT. As such, best practice Operators closely track drilling NPT. NPT is also commonly referred to as drilling flat curve time. Typical recordable NPT categories and key performance indicators used are as follows:Tight HoleDeviation ProblemsTool FailureHole Cleaning IssuesEquipment Failures and DelaysWell ControlLost Circulation Most Drilling Engineers have realized that Drilling Curves are under intense step change demands to reduce the time taken historically to reach a target depth. Tracking of NPT permits quantitative and statistical drilling analysis, to optimize drilling programs, control cost uncertainties (risks) and improve drilling economics. Drilling time is directionally proportional to cost, and in most cases, time saving (over cost) is the primary driver behind optimization strategies. Operators will often accept higher daily costs in order to save overall drilling time.

Ever increasing pressure from environmental groups and government agencies has forced many operating and service companies to take a serious look at surface casing vent flows and gas migration. Concerns regarding well abandonment and lease reclamation, as well as aquifer contamination and green house gas emissions, have increased attention to the problem. This problem has plagued the oil and gas industry for many years. Experiments using various techniques to remedy it have been costly and have consistently had low rates of success. Wireline logging, high density perforating and cement/resin squeezing have been applied. The relevant gas zones usually have very low permeability and swelling clays, preventing feedrates with water. Consequently, cement squeezing is virtually impossible due to large cement particles bridging and hydrating immediately at the perforations; often no cement is squeezed into the formation. Furthermore, shallow formations may be fragile and can fracture under a hydrostatic cement column. Multiple gas sources, source locations, and misleading log interpretation have also contributed to poor success rates. A new technique is described here which is cost effective and meets regulatory and environmental requirements. It consists of formation evaluation and the application of abrasive hydro-jetting, hydraulic fracturing, and fine particle cement squeezing. Field results in southern and central Alberta show that the technique is highly successful; surface casing vent flows and gas migration have been terminated. Introduction Whenever a hole is drilled, there is the possibility that fluids previously trapped by impermeable layers will migrate to shallower zones or to surface. Unsuccessful primary cementing may leave voids and channels in the cement sheath, allowing fluids to migrate. As a result many wells leak gas to surface through soil gas migration and/or surface casing vent flows (Figure 1). In many areas of Alberta and Saskatchewan, the first 200 to 400 m of formation consists of gravel beds, silts, undeveloped shales with swelling clays, and "non-commercial" shallow gas bearing zones. These features make it difficult to maintain borehole stability and zonal hydraulic isolation. Gas can migrate through four possible passages: between the casing and cement (microannulus), through channels in the cement, between the borehole wall and cement, and through a permeable formation (Figure 2). A surface casing vent flow is any measurable flow of gas, water, or hydrocarbon liquids with or without pressure build-up(1). It is estimated that one out of 20 wells in Alberta has pressure on the surface casing vent. Gas migration refers to gas that migrates to surface through the soil outside the surface casing, around the wellbore. Husky Oil defines a "leaker" as any well with a detectable leak, including those with rates too small to measure with standard instrumentation. A successful remedial operation is one that stops all leakage for a year or more through at least one freeze-thaw cycle(2). Elimination of gas migration and surface casing vent flows has presented a considerable challenge to the oil and gas industry. In order to abandon a lease, government reclamation requires that the site be returned to its original state.