Foreword During Underbalanced Drilling (UBD) a dilemma arises over which practice to use whenever the drill string has to be tripped out of the hole. Options are to kill the well, which may cause formation damage and defeat the object of UBD, to use a snubbing unit, or to trip with a live well, both of which have serious safety and cost implications. Setting a packer which is subsequently pushed down the hole on re-entry has been attempted on several occasions with varying degrees of success, but can only be used for completion operations and not for routine bit trips. The Downhole Deployment Valve (DDV) was developed specifically to address this issue and provides the capability to close off the well at depth, allowing the wellbore above to be de-pressurised and thereby greatly facilitating tripping operations. When the DDV is closed, reservoir pressure reaches balance point below the valve, but the wellbore above is vented, which eliminates the "pipe light" condition completely and removes the need for snubbing. The DDV is a full-bore casing valve, hydraulically controlled from surface via control lines; it is run with the casing and can be cemented in place. One onshore high pressure, high volume gas well was drilled underbalanced in early 2003 in North Eastern Thailand. The positive result from this well led to two further appraisal wells being drilled underbalanced in 2004. The DDV was used in both of these wells and this paper is a case history of the use of the DDV in Thailand, covering the following issues:DDV installation summaryDDV operating practices and proceduresTime & cost saved by the DDVImprovements suggested for subsequent wells Background The Phu Horm 3 well originally commenced operations using conventional drilling in 2002, but had to be suspended due to uncontrollable mud losses in the fractured Limestone and Dolomite reservoir. Operations re-commenced in 2003 when Phu Horm 3 became the first onshore high gas rate UBD well to be drilled in Asia Pacific. UBD not only delivered the capability to reach the well target by eliminating mud losses, but also provided a "step change" in well productivity, ensuring it would form an integral part of subsequent field development plans. Underbalanced drilling of the Phu Horm 3 well was performed without a downhole casing shut-off valve system and therefore required a snubbing unit to overcome the "pipe light" situation resulting when the drillstring weight is too low to overcome wellbore pressure resulting from UBD. In this application, the snubbing unit was used to control pipe movement in and out of the well above the pipe light point. Snubbing is slow compared to conventional tripping; resulting in extended rig time and snubbing services can be costly, with a team of up to 8 persons required for 24 hour operations. The Phu Horm 4 and 5 wells were designed as vertical wells to a TD of 2,950mMD with 16", 12_" and 8_" hole sections culminating in 7" casing being set above the reservoir. Underbalanced drilling of the reservoir was a 6" hole section, completed using a 4_" slotted liner. Reservoir pressure was expected to be 3,730psi at the top limestone with a gradient of 0.0692psi/ft and the gas stream was expected to contain 0.5% CO2 and a maximum of 5ppm H2S. A tender exercise sourced the "Downhole Casing Shut-off System" and submissions were received from interested vendors in October 2003. The tender invitation stipulated the following:A downhole casing valve capable of isolating the lower wellbore to enable tripping under atmospheric conditions.The valve should be capable of being run inside 9–5/8" casing (47lbs/ft) and 8_" open hole.The ID of the valve should be equivalent to the 7" casing string (26lbs/ft).The valve should be capable of isolating a pressure differential of 3,500 - 4,000psi.

Shales account for 75% of the formations drilled worldwide, while drilling troubles associated with shale represents 90% of wellbore stability problems reported. In this paper, a traditional wellbore stability study has been implemented at an onshore gas field in a tectonically active region. Despite following the program however, the drilling experienced significant challenges, particularly in the shales where the sonic logs have shown high velocity and therefore high rock strength. To investigate the root cause, several mechanisms for the wellbore instabilities have been investigated, such as overpressure, weak bedding plane, chemical activity, insufficient mud weight, and fractured shales. The cavings collected and preserved from the wells, as big as several inches in size, have undergone XRD, SEM, thin-section, and strength tests. While most mechanisms have been ruled out, focus has turned to the fractured shales. With the evidence from the lab and collaborated with drilling experience in the field, it has found the troublesome shale formation has been extensively fractured during past and present tectonic movements. The presence of the fissure network becomes so significant that it dominates rock mechanical behavior and wellbore stability. More interestingly, it seems the sonic logs have overlooked the existence of fissures and still shown the response from a typical strong rock. Based on those findings, modified drilling strategies have been suggested and implemented in the region. The field case presents a new challenge to the log-based wellbore stability interpretation routinely implemented in current drilling society. Being openminded, closely monitoring and testing cavings, and updating the model with drilling experience may be as important as the model development itself.