Underbalanced drilling using gasified fluids is one of the most widely used methods to drill depleted, low pressure and highly fractured formations. For ensuring a safe and successful underbalanced drilling operation, accurate prediction of the equivalent circulating density (ECD) is very important. Nevertheless, estimating ECD of gasified fluids is not easy due to the complexity of the two-phase fluid flow inside the wellbore. In this study, there are two major focuses considered; i) validation of the accuracy of Beggs& Brill (1973) model on the prediction of pressure losses of gasified fluids in under balanced drilling operation, and modification of Beggs &Brill (1973) model for pressure loss estimation inside the wellbore, and ii) to propose an ECD calculation procedure for gasified fluids by using modified Beggs & Brill (1973) model. To validate the accuracy of Beggs & Brill (1973) model, experiments were carried out using Middle East Technical University (METU) Cuttings Transport Facility to obtain the pressure losses of gasified fluids in an annulus and their corresponding flow patterns. Air-water mixtures were used with various in-situ air and water flow velocities of 0–120 ft/s and 0–10 ft/s, respectively, at wellbore inclinations of 90°, 75°, 60°, 45° and 12.5° without inner pipe rotation. Pressures were recorded at several points along the annular test section, and pressure distribution along the test section was measured. Meanwhile, flow patterns were determined by the help of a high speed digital camera. Results showed that although Beggs & Brill (1973) model can estimate pressure losses in low gas and liquid flow rates and low slip ratio between two phases for horizontal and near horizontal annular sections with a reasonable accuracy, this model cannot accurately calculate pressure losses at inclined and vertical annular sections. With some modifications, improved Beggs & Brill (1973)model (by applying suggested procedure) can be used to predict ECD and annular pressure losses of gasified fluids inside the annulus accurately. This information can be directly applied for underbalanced drilling operations when gasified fluids are used.

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Straits of Malacca Exploration Campaign marks PETRONAS' first oil/gas well drilling in the region of West Coast of Peninsular Malaysia. Three (3) exploration wells were drilled in a period of four (4) months. This paper will highlight the challenges, lessons learnt and key to success of this fast track drilling campaign in a new region of operation.The team which was entrusted with the mission to drive PETRONAS' quest of oil and gas in this new region were then given six (6) months to deliver the first well out of three (3) exploration wells. Apart from the uphill task to set up staging base and logistics support in the West Coast of Peninsular Malaysia, the team faced challenges of drilling in one of the world's busiest shipping lane. Navigational safety is one of the main concern here. Besides, two (2) of the three (3) wells were located close area where sea robberies and hijacking were rampant. In terms of drilling operation, two (2) of the wells were exposed to the risk of total losses. This paper will share the approaches taken by the project team in overcoming challenges in three (3) main areas -(1) logistics; (2) navigational and offshore safety and security; and (3) well engineering.Although faced with numerous challenges together with limitation of time and resources, the project team has managed to deliver all the three (3) wells successfully, meeting all the geological objectives within Authorisation For Expenditure (AFE) cost. On top of that, the drilling campaign was completed with zero Lost Time Incident (LTI) and zero accident. Another notable success in this project is setting up the modus operandi in a new region of drilling within six (6) months. Due to the fast track nature of this campaign, first of the three (3) wells was spudded concurrently with 3D seismic interpretation by taking the risk of relying on 2D seismic data. Halfway through the well construction, Well #1 (Well A) was re-sanctioned based on the findings while drilling and latest 3D seismic data received. Target depth of the well was revised to 3130m TVDSS from initial 2100m TVDSS. Despite all the challenges, the drilling team managed to complete drilling operation of three (3) wells ahead of time by 11 days in total.Apart from engineering and logistics challenges, this paper will share the experience of drilling in one of the busiest shipping lanes in the world. Lessons learnt and key success factors of this fast track exploration drilling campaign will be beneficial to all oil and gas (O&G) operators, especially to those planning to operate in the Straits of Malacca or any other similar regions in the world.