A Study of Formation Damage Caused by Oil–Based Mud in Dynamic Condition

Abstract

Daya pengeluaran telaga boleh terjejas dengan teruknya akibat daripada kerosakan yang berlaku berhampiran lubang telaga, berikutan aktiviti penggerudian. Dengan itu, kertas kerja teknikal ini membincangkan tentang kerosakan formasi yang dialami teras batu pasir Berea yang ditempatkan di dalam telaga tegak dan mendatar terselaku, berpunca daripada penggunaan lumpur gerudi. Kajian dilaksanakan dalam keadaan dinamik menggunakan tekanan pembezaan yang berlainan. Lumpur dasar minyak disediakan di makmal, dengan peralatan makmal standard digunakan bagi menentukan sifat reologi lumpur terbabit. Sebelum bermulanya kajian, teras batu pasir Berea ditepukan dengan sarapar selama 24 jam. Keputusan kajian dibuat berdasarkan data kehilangan turasan dan data ketertelapan teras yang diukur sebelum dan selepas sampel teras didedahkan kepada lumpur. Keputusan kajian menunjukkan bahawa kerosakan ketertelapan teras bertambah teruk dengan meningkatnya tekanan pembezaan. Walau bagaimanapun, pada tekanan pembezaan 250 psi (1723.7 kPa), kerosakan yang berlaku lebih rendah berbanding 200 psi (1379.0 kPa) bagi kedua–dua telaga mendatar dan tegak. Imej SEM menunjukkan kewujudan retakan halus pada sampel teras yang didedahkan kepada aliran lumpur dengan tekanan pembezaan dan suhu yang tinggi. Keputusan kajian juga menunjukkan bahawa telaga mendatar mengalami kerosakan yang lebih teruk dan kehilangan turasan yang lebih rendah jika dibandingkan dengan telaga tegak. Kata kunci: Nisbah kerosakan, aliran dinamik, kerosakan formasi, lumpur dasar minyak, SEM Well productivity can be significantly affected by damage near to wellbore area caused by drilling. Thus, this technical paper discusses the formation damage caused by oil–based mud on the Berea sandstone cores in simulated vertical and horizontal wells. This study was carried out in a dynamic condition at various differentials pressure. Oil–based mud was prepared in the laboratory and standard equipments were used to determine the mud rheological properties. Berea sandstone cores were saturated with sarapar for 24 hours prior to the tests. The result of this study was based on the measurement of filtrate loss and core permeability measurement before and after exposure to oil–based mud in vertical and horizontal conditions at various differentials pressure. The experimental results revealed that the permeability impairment increased as differential pressure increases. However, at differential pressure of 250 psi (1723.7 kPa), the damage occurred was less severe compared to 200 psi (1379.0 kPa), in both vertical and horizontal conditions. SEM image showed the presence of microfractures in the core when exposed to mud at high pressure and temperature, under dynamic condition. Experimental results also showed that the horizontal condition experienced more severe damage amd lower filtrate loss as compared to vertical condition. Key words: Damage ratio, dynamic flow, formation damage, oil-based mud, SEM